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The Principles of Readability

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A brief introduction to the research on readability (reading ease) and the readability formulas. Readability is tightly related to reading comprehension, retention, reading speed, and persistence. The readability formulas use variables that are known to be among the first causes of reading difficulty. While there are many other features of language they do not measure, the readability formula scores correlate .90 and above with comprehension as measured by reading tests. They have benefited millions of readers world-wide in many languages.
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The Principles of
William H. DuBay
Copyright © 2004 William H. DuBay Page ii
The Principles of Readability
25 August 2004
2004 William H. DuBay. All Rights Reserved.
The Principles of Readability gives a short history of literacy studies in the U.S.
and a short history of research in readability and the readability formulas.
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William DuBay
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Phone: (949) 631-3309
Copyright © 2004 William H. DuBay Page iii
Introduction .......................................................................................................... 1
Guidelines For Readability.......................................................................... 2
The readability formulas..............................................................................2
Are the readability formulas a problem? .....................................................2
What is readability?..................................................................................... 3
Content ........................................................................................................3
The Adult Literacy Studies................................................................................... 4
Grading the reading skills of students..........................................................4
Grading adult readers ..................................................................................4
U.S. military literacy surveys—reading on the job...................................... 4
U.S. civilian literacy surveys....................................................................... 6
Challenges for technical communicators..................................................... 9
The Classic Readability Studies ......................................................................... 10
L. A. Sherman and the statistical analysis of literature.............................. 10
Vocabulary-Frequency Lists...................................................................... 11
The Classic Readability Formulas............................................................. 13
The New Readability Studies ............................................................................. 25
A Community of Scholars ......................................................................... 26
The Cloze Test ..........................................................................................27
Reading Ability, Prior Knowledge, Interest, and Motivation .................... 28
Reading Performance ................................................................................ 30
The Measurement of Content ....................................................................31
Text Leveling ............................................................................................35
Producing and Transforming Text............................................................. 37
The New Readability Formulas ................................................................. 43
Formula Applications................................................................................ 55
Using the Formulas....................................................................................56
References .......................................................................................................... 59
Biosketch............................................................................................................ 72
The Principles of Readability
Copyright © 2004 William H. DuBay
Page iv
Copyright © 2004 William H. DuBay Page 1
Over 80 years of research and testing have
contributed to the worldwide use in many
languages of the readability formulas. They help
us improve the text on the level of words and
sentences, the first causes of reading difficulty.
The principles of readability are in
every style manual. Readability formulas
are in every word processor. What is
missing is the research and theory on which they stand.
The Principles of Readability
By William H. DuBay
In 1998, traffic accidents caused 46 percent of all accidental deaths of infants
and children aged 1 to 14 (National Center for Health Statistics, 2000). One
study (Johnston et al. 1994) showed that the single strongest risk factor for injury
in a traffic accident is the improper use of child-safety seats. Another study
(Kahane 1986) showed that, when correctly used, child safety seats reduce the
risk of fatal injury by 71 percent and hospitalization by 67 percent.
To be effective, however, the seats must be installed correctly. Other studies,
showed that 79 to 94 percent of car seats are used improperly (National Highway
Traffic Safety Administration 1996, Decina and Knoebel 1997, Lane et al.
Public-health specialists Dr. Mark Wegner and Deborah Girasek (2003)
suspected that poor comprehension of the installation instructions might
contribute to this problem. They looked into the readability of the instructions
and published their findings in the medical journal Pediatrics. The story was
covered widely in the media.
The authors referred to the National Adult Literacy Study (National Center for
Educational Statistics, 1993), which states the average adult in the U.S. reads at
the 7th grade level. They also cited experts in health literacy who recommend that
materials for the public be written at the fifth or sixth-grade reading level (Doak
et al., 1996; Weiss and Coyne, 1997).
Their study found that the average reading level of the 107 instructions they
examined was the 10th grade, too difficult for 80 percent adult readers in the U.S.
When texts exceed the reading ability of readers, they usually stop reading. The
authors did not address the design, completeness, or the organization of the
instructions. They did not say that the instructions were badly written. Armed
with the SMOG readability formula, they found the instructions were written at
the wrong grade level. You can be sure the manufacturers of the car safety seats
are scrambling to re-write their instructions.
The Principles of Readability
Copyright © 2004 William H. DuBay
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Guidelines For Readability
In works about technical communication, we are often told how to avoid such
problems. For example, JoAnn Hackos and Dawn Stephens in Standards for
Online Communication (1997) ask us to “conform to accepted style standards.”
They explain:
Many experts, through much research, have compiled golden rules of
documentation writing. These rules apply regardless of medium:
Use short, simple, familiar words
Avoid jargon.
Use culture-and-gender-neutral language.
Use correct grammar, punctuation, and spelling.
Use simple sentences, active voice, and present tense.
Begin instructions in the imperative mode by starting sentences with
an action verb.
Use simple graphic elements such as bulleted lists and numbered
steps to make information visually accessible.
For more suggestions, we recommend referring to one of many
excellent books on writing style, especially technical style.
We all know of technical publications that do not follow these guidelines and are
read only by a small fraction of the potential readership. One reason may be that
the writers are not familiar with the background and research of these guidelines.
This paper looks most carefully at two of the most important elements of
communication, the reading skills of the audience and the readability of the text.
The readability formulas
In the 1920s, educators discovered a way to use vocabulary difficulty and
sentence length to predict the difficulty level of a text. They embedded this
method in readability formulas, which have proven their worth in over 80 years
of application.
Progress and research on the formulas was something of a secret until the 1950s.
Writers like Rudolf Flesch, George Klare, Edgar Dale, and Jeanne Chall brought
the formulas and the research supporting them to the marketplace. The formulas
were widely used in journalism, research, health care, law, insurance, and
industry. The U.S. military developed its own set of formulas for technical-
training materials.
By the 1980s, there were 200 formulas and over a thousand studies published on
the readability formulas attesting to their strong theoretical and statistical
Are the readability formulas a problem?
In spite of the success of the readability formulas, they were always the center of
controversy. When the “plain language” movement in the 1960s resulted in
legislation requiring plain language in public and commercial documents a
number of articles attacked the use of readability formulas. They had titles like,
“Readability: A Postscript” (Manzo 1970), “Readability: Have we gone too far?”
(Maxwell 1978), “Readability is a Four-letter Word” (Selzer 1981), “Why
Readability Formulas Fail” (Bruce et al. 1981), “Readability Formulas: Second
Looks, Second Thoughts“ (Lange 1982), “Readability Formulas: What’s the
Use?” (Duffy 1985) and “Last Rites for Readability Formulas in Technical
Communication” (Connaster 1999).
The Principles of Readability
Copyright © 2004 William H. DuBay
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Many of the critics were honestly concerned about the limitations of the formulas
and some of them offered alternatives such as usability testing. Although the
alternatives are useful and even necessary, they fail to do what the formulas do:
provide an objective prediction of text difficulty.
Although the concerns of the formula critics have been amply addressed
elsewhere (Chall 1984, Benson 1984-1985, Fry 1989b, Dale and Chall 1995,
Klare 2000), we will examine them again in some detail, with a special regard
for the needs of technical communication.
The purpose of this article is to very briefly review the landmark studies on
readability and the controversy regarding the formulas. I will be happy if you
learn something of the background of the formulas, what they are good for, and
what they are not. That knowledge will give you greater confidence and method
in tailoring your text for a specific audience.
What is readability?
Readability is what makes some texts easier to read than others. It is often
confused with legibility, which concerns typeface and layout.
George Klare (1963) defines readability as “the ease of understanding or
comprehension due to the style of writing.” This definition focuses on writing
style as separate from issues such as content, coherence, and organization. In a
similar manner, Gretchen Hargis and her colleagues at IBM (1998) state that
readability, the “ease of reading words and sentences,” is an attribute of clarity.
The creator of the SMOG readability formula G. Harry McLaughlin (1969)
defines readability as: “the degree to which a given class of people find certain
reading matter compelling and comprehensible.” This definition stresses the
interaction between the text and a class of readers of known characteristics such
as reading skill, prior knowledge, and motivation.
Edgar Dale and Jeanne Chall’s (1949) definition may be the most
comprehensive: “The sum total (including all the interactions) of all those
elements within a given piece of printed material that affect the success a group
of readers have with it. The success is the extent to which they understand it,
read it at an optimal speed, and find it interesting.”
Beginning early in the last century in the U.S., studies of the reading ability of
adults and the readability of texts developed in tandem. Our subject matter falls
under these headings:
The Adult Literacy Studies These studies discovered great differences in
the reading skills of adults in the U.S. and their implications for society.
The Classic Readability Studies This section looks at the early readability
studies, which started in the late 19th century and concluded in the
1940s, with the publication of the popular Flesch and Dale-Chall
formulas. During this period, publishers, educators, and teachers were
concerned with finding practical methods to match texts to the skills of
readers, both students and adults.
The New Readability Studies Beginning in the 1950s, new developments
transformed the study of readability, including a new test of reading
comprehension and the contributions of linguistics and cognitive
psychology. Researchers explored how the reader’s interest, motivation,
and prior knowledge affect readability. These studies in turn stimulated
the creation of new and more accurate formulas.
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Copyright © 2004 William H. DuBay
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The Adult Literacy Studies
Grading the reading skills of students
Before the mid-19th century, schools in the U.S. did not group students according
to grade. Students learned from books that their families had, often Bibles and
hornbooks. American educator Horace Mann, who had studied the supervision,
graded classes, and well-articulated classes of Prussian schools, struggled to
bring those reforms to America.
It was not until 1847 that the first graded school opened in Boston with a series
of books prepared for each grade. Educators found that students learn reading in
steps, and they learn best with materials written for their current reading level.
Since then, grouping by grades has functioned as an instructional process that
continues from the first year of school through high school and beyond.
Although reading standards were set for each grade, we know that not all
students in the same class read at the same level. A 7th-grade teacher, for
example can typically face a classroom of students with reading ability from the
2nd to the 12th grade. Good teaching practice has long separated students in the
same class by reading ability for separate instruction (Betts 1946, Barr and
Dreeben 1984).
Educators promoted the target reading levels for each class with the use of
standardized reading tests. William A. McCall and Lelah Crabbs (1926) of the
Teachers College of Columbia University published Standard Test Lessons in
Reading. Revised in 1950, 1961, and 1979, these tests became an important
measure of the reading ability of students in the U.S. These and later reading
tests typically measure comprehension by having students first read a passage
and then answer multiple-choice questions.
The Mc Call-Crabbs reading tests also became important in the development and
validation of the readability formulas. Later reading tests also used for creating
and testing formulas for adults and children include the Gates-MacGinitie
Reading Tests, the Stanford Diagnostic Reading Test, the California Reading
Achievement Test, the Nelson-Denny Reading Test, the Diagnostic Assessment
of Reading with Trial Teaching Strategies and the National Assessment of
Educational Progress (NAEP).
Grading adult readers
For a long time, no one thought of grading adults, who were considered either
literate or illiterate. This began to change with the first systematic testing of
adults in the U.S. military in 1917. The testing of civilians began in Chicago in
During that first period, investigators discovered that general readers in the U. S.
were adults of limited reading ability. The average adult was able to read with
pleasure nothing but the simplest adult materials, usually cheap fiction or
graphically presented news of the day.
Educators, corporations, and government agencies responded by providing more
materials at different reading levels for adults.
U.S. military literacy surveys—reading on the job
General George Washington first addressed concerns about the reading skills of
fighters during the Revolutionary War. He directed chaplains at Valley Forge to
teach basic skills of reading, writing, and arithmetic to soldiers. Since then, the
U.S. armed services has invested more in studying workplace literacy than any
other organization.
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Copyright © 2004 William H. DuBay
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Since the 50s in the U.S., you have to pass a literacy test to join the Armed
Services. From such a test and others, the military learns a lot about your
aptitudes, cognitive skills, and ability to perform on the job.
It took a while for the military to develop these tests. Over the years, it changed
the content of the tests and what they measure. Testing literacy advanced in these
general stages:
1. During World War I, they focused on testing native intelligence.
2. The military decided that what they were testing was not so much raw
intelligence as reading skills. By World War II, they were focusing on
classifying general learning ability for job placement.
3. In the 1950s, Congress mandated a literacy requirement for all the
armed services. The resulting Armed Forces Qualification Test
(AFQT) prevented people of the lowest 10% of reading ability from
entering military service. The military then combined AFQT subtest
with other tests, which differed for each service and sorted recruits into
different jobs.
4. In 1976, with the arrival of the All-Volunteer Force, the military
introduced the Armed Services Vocational Aptitude Battery
(ASVAB). All military services used this test battery for both screening
qualified candidates and assessing trainability for classified jobs.
5. In 1978, an error resulted in the recruitment of more than 200,000
candidates in the lowest 10% category. The military, with the aid of
Congress, decided to keep them. The four military services each created
workplace literacy programs, with contract and student costs over $70
million. This was a greater enrollment in adult basic education than in
all such programs of 25 states combined. The results of the workplace
literacy programs were considered highly successful, with performance
and promotions “almost normal.”
6. In 1980, the military further launched the largest study ever in job
literacy, the Job Performance Measurement/Enlistment Standards
Project. They invested $36 million in developing measures of job
performance. Over ten years, the project involved more than 15,000
troops from all four military services. Dozens of professionals in
psychological measurement took part in this study.
7. In 1991, based on these findings, the military raised its standards and
combined the ASVAB with the AFQT and special aptitude tests from
all the services into one battery of 10 tests. Both the Army and Navy
continue to provide workplace-literacy programs for entering recruits
and for upgrading the literacy skills of experienced personnel (Sticht
1995, pp 37-38).
The major findings of the military research were:
1. Measures of literacy correlate closely with measures of intelligence and
2. Measures of literacy correlate closely with the breadth of one’s
3. Measures of literacy correlate closely to job performance. Hundreds of
military studies found no gap between literacy and job performance.
4. Workplace literacy programs are highly effective in producing, in a
brief period, significant improvements in job-related reading.
The Principles of Readability
Copyright © 2004 William H. DuBay
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5. Advanced readers have vast bodies of knowledge and perform well
across a large set of domains of knowledge. Poor readers perform
poorly across these domains of knowledge. This means that, if
programs of adult literacy are to move students to high levels of
literacy, they must help them explore and learn across a wide range of
knowledge (Sticht and Armstrong 1994, pp. 37-38).
The military studies indicated that achieving high levels of literacy requires
continued opportunities for life-long learning. Investments in adult literacy
provide a unique and cost-effective strategy for improving the economy, the
home, the community, and the schools.
U.S. civilian literacy surveys
University of Chicago Study Guy Buswell (1937) of the University of Chicago
surveyed 1,000 adults in Chicago with different levels of education. He
measured skills in reading materials such as food ads, telephone directories, and
movie ads. He also used more traditional tests of comprehension of paragraphs
and vocabulary.
Buswell found that reading skills and practices increase as years of education
increase. He suggested that an important role of education is to guide readers to
read more, and that reading more leads to greater reading skill. In turn, this may
lead one to continue more education, thus leading to greater reading skill.
Fig. 1. Adult literacy in 1937. This study confirmed the relationship between reading
skill and years of education completed. Sources:
Buswell, G. 1937 pp. 27, 57, 71).
The National Assessment of Educational Progress (NAEP) of 1970-1971
This study tested how students 9, 13, and 17 years old as well adults 26 to 35
years old perform on 21 different tasks. The results showed for the first time how
age affects performance on the same items. This survey showed as children grow
up, attend school, and become adults, they grow progressively more literate
(Sticht and Armstrong, pp. 51-58).
Louis Harris survey of 1970 The Louis Harris polling organization surveyed
adults representing a cross section of the U.S. population. The subjects filled out
five common application forms, including an application for a driver’s license
and a Medicaid application.
The poll was the first of many to show that many U.S. citizens have difficulty
with filling out forms. The Medicaid form was difficult, with only 54 percent of
The Principles of Readability
Copyright © 2004 William H. DuBay
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those with an 8th grade education or less getting 90-100 percent correct. Even
many college-educated adults had trouble completing the Medicaid form (Sticht
and Armstrong, pp. 59-62).
Adult Functional Reading Study of 1973 This study used household
interviews to find out the literacy practices of adults. It used a second household
sample to assess literacy skills.
Over all 170 items used in the study, over 70 percent of the respondents scored
70 percent correct or better. As a trend, adults with more education performed
better on the test than those with less.
As with Buswell's study, both literacy skills and literacy practices correlated
closely with education. Book and magazine reading correlated more closely with
years of education than did newspaper reading. Altogether, the adults reported
that they spent about 90 minutes a day in reading materials such as forms, labels,
signs, bills, and mail. (Sticht and Armstrong, pp. 63-66).
Adult Performance Level Study of 1971 This study began as a project funded
by the U. S. Office of Education. It introduced "competency-based" education,
directing adult education to focus on achieving measurable outcomes. By 1977,
two-thirds of the states had set up some form of "competency-based" adult basic
The test included over 40 common and practical tasks, such as filling out a
check, reading the want ads, addressing an envelope, comparing advertised
products, filling out items on a 1040 tax form, reading a tax table, and filling out
a Social Security application. Results showed the high correlation between
performance on all tasks and literacy (Sticht and Armstrong, pp. 67-98).
What a Reading Grade Level Means
The reading grade level of a text depends on the use of the text. If the text is
used for independent, unassisted, or recreational use, the reading grade level
will be higher than a text destined for classroom use and optimum learning
gain. In other words, the same text will be easier for those with more advanced
reading skills (with a higher grade level) and harder for those with less (and with
a lower grade level). See the “Problem of Optimal Difficulty” below.
The grade of completed education is no indication of one’s reading level.
Average high-school graduates read at the 9th-grade level, which means a large
number reads below that level. Those who pursue special domains of
knowledge may develop higher levels of reading skill in those specialties than
they have for general reading. Thus, college graduates, who prefer to read
general materials at the 10th-grade level, may prefer more difficult texts within
their own specialty. Students who are poor readers of general classroom
material are often able to master difficult treatments of subjects that appeal to
Young Adult Literacy Survey of 1985 This study of young adults (17-25) and
the adult study that followed in 1992 both measured the literacy the same way in
three areas:
Prose literacy—meaning of selected texts
Document literacy—finding information on a form such as a bus
Quantitive literacy—mathematical and spatial tasks.
Both studies used a literacy scoring range of 1 to 500 and the five levels of skill
defined by the National Assessment of Educational Progress (1985). John
Carroll (1987) estimated the corresponding reading-grade levels as shown in
Table 1.
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Copyright © 2004 William H. DuBay
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NAEP Level Literacy Score Grade Level
I Rudimentary 150 1.5
II Basic 200 3.6
III Intermediate 250 7.2
IV Adept 300 12
V Advanced 350 16+
Table 1. NAEP proficiency levels and the reading-grade-level
The young adult survey by the NAEP (1985) found that only 40 percent of
young adults 17 to 25 no longer in high school, and 17 years old and in high
school, read at a 12th-grade level. Large numbers leave high school still reading
at the 8th-grade level or lower. The 1990 census showed that 24.8 percent of
adults did not graduate from high school.
The National Adult Literacy Survey (NALS) of 1992 This U.S. Government
study sampled 26,000 adults, representing 191 million adults. In 1993, it
published the first of a number of reports on this survey entitled, "Adult Literacy
in America” (National Center for Education Statistics 1993, 1999, 2001).
This study used the same tests as the Young Adult Literacy Survey and reported
data with the same five levels of skill.
Literacy Skill Level 1 Level 2 Level 3 Level 4 Level 5
Prose 21% 27% 32% 17% 3%
Document 23% 28% 31% 15% 3%
Quantitative 22% 25% 31% 17% 4%
Table 2. Percentages of adults in the U.S. in each of the five NAEP skill levels for each
literacy skill (Sticht and Armstrong 1995, p. 113).
The data in this table suggest 40 to 44 million adults in the U.S. are in Level 1,
defined as “functionally illiterate, not having enough reading skills for daily
life.” Some 50 million are in Level 2. This means the percentage of adults who
struggle at Levels 1 and 2 (below the 5th-grade level) in the U.S. reaches 48
The report confirmed that numeracy (quantitative) skills increase with reading
skills. Adults of different reading skills not only have different worldviews but
also different life experiences. Forty-three percent of adults with low-literacy
skills live in poverty, 17% receive food stamps, and 70% have no job or part-
time job. Over 60 % of frontline workers producing goods have difficulty
applying information from a text to a task. More than 20% of adults read below
the sixth-grade level, far below the level needed to earn a living wage.
Adults at Level 1 earned a median income of $240 a week, while those at Level
5 earned $681. Seventy percent of prisoners are in the lowest two levels.
In support of these figures, the number of companies reporting shortages of
skilled workers doubled between 1995 and 1998. Ninety percent of Fortune
1000 executives reported that low literacy is hurting productivity and
profitability. In one survey, more than half of the responding company
representatives said that high school graduates applying for jobs are not literate
enough to hire.
Low levels of literacy have caused costly and dangerous mistakes in the
workplace. There are other costs in billions of dollars in the workplace resulting
from low productivity, poor quality of products and services, mistakes,
absenteeism, and lost management time.
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Copyright © 2004 William H. DuBay
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The Adult Literacy Survey also confirmed the effects of literacy on health care.
Since 1974, when health officials became aware of the effects of low literacy on
health, literacy problems have grown. A more complex health-care system
requires better reading skills to negotiate the system and take more responsibility
for self-care.
Using a nationally representative sample of the U.S. adult population age 16 and
older, the National Academy (2002) on an Aging Society examined the impact
of literacy on the use of health care services. The study found that people with
low health-literacy skills use more health care services.
Among adults who stayed overnight in a hospital in 1994, those with low health
literacy skills averaged 6 percent more hospital visits, and stayed in the hospital
nearly 2 days longer than adults with higher health literacy skills. The added
health-care costs of low literacy are estimated at $73 billion in 1998 dollars. This
includes $30 billion for the Level 2 population plus $43 billion for the Level 1
population. The total is about what Medicare pays for doctor services, dental
services, home health care, prescription drugs, and nursing-home care combined.
Low literacy is not chiefly the problem of immigrants, the elderly, high school
dropouts, or people whose first language is not English. Low literacy is a
problem that knows no age, education, income levels, or national origins. Most
people with low literacy skills were born in this country and have English as
their first language.
One solution to the problem of low literacy of adults is more government and
corporate support for adult literacy programs. Workplace literacy programs have
cost-effective and lasting results. Another solution is to produce more texts that
are written for people of diverse reading skills.
Challenges for technical communicators
The lessons of the literacy studies for technical communicators are obvious:
Low and intermediate literacy skills are a big problem for large
numbers of users of technical documents. Providing technical
documents at their levels will advance both their technical and reading
The larger the audience, the more it will include the average reading
habits and skills of the public as determined by the literacy surveys.
The more critical the information is for safety and health, the greater is
the need for increased readability.
The finding that the great majority of adult readers are mid-range, intermediate
readers brings to us in technical communication new opportunities and
Intermediate readers represent a large audience that technical documents have
been missing. Go into any library or bookstore, and you will find few technical
or scientific publications in the “Young Adult” section, or elsewhere written at
the 7th to 9th-grade level. On the Internet, there is the same scarcity of
intermediate technical materials.
For example, a small sampling of the author’s shows that the support sections of
the Apple and Microsoft Web sites are written at advanced level of 10th grade
and up. The technical books for Dummies and Idiots, while written in a casual
style, are often at the 10th-grade level and up. Like the car-safety seat
instructions, these technical documents are too difficult for 80 percent of adult
readers in the U.S. Ironically, the user manual that comes with the CorelDraw
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 10
program is written at the 7th-grade level, making it fit for a much larger audience
than its Dummies counterpart.
Considering the keen interest that intermediate readers of all ages can have in
technical matters, this literacy gap is troubling. While some highly motivated
readers are able to master difficult technical materials, we cannot assume that
everyone will do so. To the contrary, the difficulty of technical materials has
taught many if not most readers of intermediate skill not to look for technical
help in written texts. Helpful text means not only providing readers accurate
information but also information written at the reading levels they need.
The Classic Readability Studies
The first aim of the classic readability studies was to develop practical methods
to match reading materials with the abilities of students and adults. These efforts
centered on making easily applied readability formulas which teachers and
librarians could use.
The first adult literacy surveys in the U.S. in the 1930s brought new concerns
about providing graded texts for adults. For the rest of the century, publishers,
librarians, teachers, and investigators addressed that need with new methods of
determining the reading level of texts.
The classic readability studies include these landmark issues:
L. A. Sherman and the statistical analysis of literature.
The vocabulary-frequency lists
The classic readability formulas
L. A. Sherman and the statistical analysis of literature
Down through the centuries, many had written about the differences between an
“ornate” and “plain” style in English.
In 1880, a professor of English Literature at the University of Nebraska, Lucius
Adelno Sherman, began to teach literature from a historical and statistical point
of view.
He compared the older prose writers with more popular modern writers such as
Macaulay (The History of England) and Ralph Waldo Emerson. He noticed a
progressive shortening of sentences over time.
He decided to look at this statistically and began by counting average sentence
length per 100 periods. In his book (1893), Analytics of Literature, A Manual for
the Objective Study of English Prose and Poetry, he showed how sentence-
length averages shortened over time:
Pre-Elizabethan times: 50 words per sentence
Elizabethan times: 45 words per sentence
Victorian times: 29 words per sentence
Sherman’s time: 23 words per sentence.
In our time, the average is down to 20 words per sentence.
Sherman’s work set the agenda for a century of research in reading. It proposed
the following:
Literature is a subject for statistical analysis.
Shorter sentences and concrete terms increase readability.
Spoken language is more efficient than written language.
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Over time, written language becomes more efficient by becoming more
like spoken language.
Sherman also showed how individual writers are remarkably consistent in their
average sentence lengths. This consistency was to become the basis for the
validity of using samples of a text rather than the whole thing for readability
Sherman was the first to use statistical analysis for the task of analyzing
readability, introducing a new and objective method of literary criticism.
Another of Sherman’s discoveries was that over time sentences not only became
shorter but also simpler and less abstract. He believed this process was due to the
influence of the spoken language on written English. He wrote (p. 312):
Literary English, in short, will follow the forms of the standard spoken
English from which it comes. No man should talk worse than he writes,
no man writes better than he should talk…. The oral sentence is clearest
because it is the product of millions of daily efforts to be clear and
strong. It represents the work of the race for thousands of years in
perfecting an effective instrument of communication.
Linguistic research later confirmed Sherman’s view of the relationship between
spoken and written language. Rudolf Flesch (1946) wrote that English is
following written Chinese in making language simpler by substituting standard
word order (subject-verb-object) for more complex grammar.
According to Flesch, Chinese is “the most grown-up talk of mankind. It is the
way people speak who started to simplify their language thousands of years
Fig.2. In Analytics of Literature, L.A.
Sherman looked at literature statistically.
He showed the importance of average
sentence length and the relationship
between spoken and written English.
ago and have kept at it ever since….
(p. 12).
“Among the world’s great languages, the
runner-up to Chinese is English. It’s
simpler, more flexible, more practical than
any other Western language because it has
gone furthest in losing inflections and
straightening out irregularities” (p. 20)
Sherman’s most important point was the
need to involve the reader. He wrote:
The universally best style is not a thing
of form merely, but must regard the
expectations of the reader as to the spirit
and occasion of what is written. It is not
addressed to the learned, but to all
minds. Avoiding book-words, it will use
only the standard terms and expressions
of common life… It will not run in long
and involved sentences that cannot
readily be understood. Correct in all
respects, it will not be stiff; familiar, but
safely beyond all associations of
vulgarity (p. 327).
Vocabulary-Frequency Lists
During the 1920s, two major trends stimulated a new interest in readability:
1. A changing school population, especially an increase in “first
generation” secondary school students, the children of immigrants.
Teachers reported that these students found textbooks too difficult.
The Principles of Readability
Copyright © 2004 William H. DuBay
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2. The growing use of scientific tools for studying and objectively
measuring educational problems.
One such tool, Thorndike’s Teacher’s Word Book (1921), was the first extensive
listing of words in English by frequency. It provided teachers with an objective
means for measuring the difficulty of words and texts. It laid the foundation for
almost all the research on readability that would follow.
Its author, psychologist Edward. L. Thorndike of Columbia University, noticed
that teachers of languages in Germany and Russia were using word counts to
match texts with students. The more frequent a word is used, they found, the
more familiar it is and the easier to use. As we learn and grow, our vocabulary
grows as does our ability to master longer and more complex sentences. How
much that continues to grow depends on how much reading is done throughout
A vocabulary test on the meaning of words is the strongest predictor of verbal
and abstract intellectual development. The knowledge of words has always been
a strong measure of a reader’s development, reading comprehension, and verbal
intelligence. Chall and Dale (1995, p. 84) write, “It is no accident that
vocabulary is also a strong predictor of text difficulty.”
It happens that the first words we learn are the simplest and shortest. These first,
easy words are also the words we use most frequently. Most people do not
realize the extent of this frequency. Twenty-five percent of the 67,200 words
used in the 24 life stories written by university freshmen consisted of these ten
words: the, I, and, to, was, my, in, of, a, and it (Johnson, 1946). The first 100
most frequent words make up almost half of all written material. The first 300
words make up about 65 percent of it (Fry et al, 1993).
Around 1911, Thorndike began to count the frequency of words in English texts.
In 1921, he published The Teacher’s Word Book, which listed 10,000 words by
frequency of use. In 1932, he followed up with A Teacher’s Word Book of
20,000 Words, and in 1944 with Irving Lorge, A Teacher’s Word Book of 30,000
Until computers came along, educators, publishers, and teachers commonly used
word-frequency lists to evaluate reading materials for their classes. Thorndike’s
work also was the basis for the first readability formulas for children’s books.
After Thorndike, there was extensive research on vocabulary. The high mark
came in Human Behavior and The Principle of Least Effort by Harvard’s
George Kingsley Zipf (1949).
Zipf used a statistical analysis of language to show how the principle of least
effort works in human speech. Zipf showed that, in many languages, there is a
mathematical relationship between the hard and easy words, now called Zipf’s
curve. This notion of saving energy is a central feature of language and is one of
the principle bases of research on the frequency of words.
Klare (1968), reviewing the research on word frequency, concludes: “Not only
do humans tend to used some words much more often than others, they recognize
more frequent words more rapidly than less frequent, prefer them, and
understand and learn them more readily. It is not surprising, therefore, that this
variable has such a central role in the measurement of readability.”
Dale and O’Rourke: the words Americans know In 1981, publishers of the
World Book Encyclopedia published The Living Word Vocabulary: A National
Vocabulary Inventory by Edgar Dale and Joseph O’Rourke. The authors based
this work on the earlier work of Thorndike and others as well as on a 25-year
study of their own. It contained the grade-level scores of the familiarity of
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 13
44,000 words. For the first time, it gave scores for each of the meanings a word
can have and the percentage of readers in the specified grade who are familiar
with the word.
The authors obtained the familiarity scores by giving a three-choice test to
students from the 4th to the 16th grade in schools and colleges throughout the
U.S. The editors of the encyclopedia also used the scores to test the readability
of the articles they published. Field tests of the encyclopedia later confirmed the
validity of the word scores. This work is exceptional in every respect and is
considered by many to be the best aid in writing for a targeted grade level.
Fig. 3. Sample entries from The Living Word Vocabulary. This
work featured not only grade level and a short definition, but
also the percentage of readers in that grade who know the
word. The editors of World Book Encyclopedia used this
information as one of the reading-level tests for their entries
(Dale and O’Rourke 1981).
In the preface, the Editorial Director of the encyclopedia W. H. Nault wrote (p.
v) that this work marked “the beginning of a revolutionary approach to the
preparation and presentation of materials that fit not only the reading abilities,
but the experience and background of the reader as well.”
Although this work is out of print, you can find it at libraries and used bookshops
along with other graded vocabularies and word-frequency lists such as The
American Heritage Word Frequency Book.
The Classic Readability Formulas
Harry D. Kitson—Different readers, different styles Psychologist Harry D.
Kitson (1921) published The Mind of the Buyer, in which he showed how and
why readers of different magazines and newspapers differed from one another.
Although he was not aware of Sherman’s work, he found that sentence length
and word length measured in syllables are important measures of readability.
Rudolph Flesch would incorporate both these variables in his Reading Ease
formula 30 years later.
Although Kitson did not create a readability formula, he showed how his
principles worked in analyzing two newspapers, the Chicago Evening Post and
the Chicago American and two magazines, the Century and the American. He
analyzed 5000 consecutive words and 8000 consecutive sentences in the four
publications. His study showed that the average word and sentence length were
shorter in the Chicago American newspaper than in the Post, and the American
magazine’s style simpler than the Century’s, accounting for the differences in
their readership.
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 14
The first readability formula Bertha Lively and S. L. Pressey (1923) were
concerned with the practical problem of selecting science textbooks for junior
high school. The books were so overlaid with technical words that teachers spent
all class time teaching vocabulary. They argued that it would be helpful to have a
way to measure and reduce the “vocabulary burden” of textbooks.
Their article featured the first children’s readability formula. It measured the
number of different words in each 1,000 words and the number of words not on
the Thorndike list of 10,000 words. Their method produced a correlation
coefficient of .80 when tested on 700 books.
In reading research, investigators look for correlations instead of causes. A
correlation coefficient (r = ) is a descriptive statistic that can go from +1.00 to
0.0 or from 0.0 to –1.00. Both +1.00 and –1.00 represent a perfect correlation,
depending on whether the elements are positively or negatively correlated.
A coefficient of 1.00 shows that, as one element changes, the other element
changes in the same (+) or opposite (-) direction by a corresponding amount. A
coefficient of .00 means no correlation, that is, no corresponding relationship
through a series of changes.
For example, if a formula should predict a 9th-grade level of difficulty on a 7th-
grade text, and, if at all grade levels, the error is in the same direction and by a
corresponding amount, the correlation could be +1.00 or at least quite high. If,
on the other hand, a formula predicts a 9th-grade level for a 6th-grade text, an 8th
grade level for a 10th-grade text, and has similar variability in both directions, the
correlation would be very low, or even 0.00.
Squaring the correlation coefficient ( r2 = ) gives the percentage of
accountability for the variance. For example, the Lively and Pressey formula
above accounts for 64% (.802) of the variance of the text difficulty.
Other early school formulas Mabel Vogel and Carleton Washburne (1928) of
Winnetka, Illinois carried out one of the most important studies of readability.
They were the first to study the structural characteristics of the text and the first
to use a criterion based on an empirical evaluation of text. They studied ten
different factors including kinds of sentences and prepositional phrases, as well
as word difficulty and sentence length. Since, however, many factors correlated
highly with one another, they chose four for their new formula.
Following Lively and Pressey, they validated their formula, called the Winnetka
formula, against 700 books that had been named by at least 25 out of almost
37,000 children as ones they had read and liked. They also had the mean reading
scores of the children, which they used as a difficulty measure in developing
their formula. Their new formula correlated highly ( r = .845) with the reading
test scores.
With this formula, investigators knew that they could objectively match the
grade level of a text with the reading ability of the reader. The match was not
perfect, but it was better than subjective judgments. The Winnetka formula, the
first one to predict difficulty by grade levels, became the prototype of modern
readability formulas.
Vogel and Washburne’s work stimulated the interest of Alfred S. Lewerenz
(1929, 1929a, 1935, 1939), who produced several new readability formulas for
the Los Angeles School District.
W. W. Patty and W. I. Painter (1931) discovered the year of highest burden in
high school is the sophomore year. They also developed a formula to measure
the relative difficulty of textbooks based on a combination of frequency as
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 15
determined by the Thorndike list and vocabulary diversity (the number of
different words in a text).
With the rise of the plain-language movement in the 1960s, several critics of the
formulas claimed that the formulas do not test comprehensibility (Kern 1979,
Duffy and Kabance 1981, Duffy 1985). The research, however, shows that from
the beginning their scores correlate well with comprehension difficulty as
measured by reading tests. The formulas rate very well when compared with
other widely used psychometric measurements such as reading tests (Chall and
Dale 1995). Their validity correlations make them useful for predicting the
comprehension difficulty of texts (Bormuth 1966).
Waples and Tyler: What adults read During the Depression in the ‘30s, adult
education and the increased use of libraries stimulated studies in reading.
Sociologists studied “who reads what and why over consecutive periods,”
looking at reading as an aspect of mass communication.
Douglas Waples and Ralph W. Tyler (1931) published What People Want to
Read About, a comprehensive, two-year study of adult reading interests. Instead
of using the traditional library circulation records to determine reading patterns,
they interviewed people divided by sex and occupation into 107 different groups.
It showed the types and styles of materials that people not only read but also
want to read. It also studied what they did not read and why.
They found that the reading of many people is limited because of the lack of
suitable material. Readers often like to expand their knowledge, but the reading
materials in which they are interested are too difficult.
Ralph Ojemann: The difficulty of adult materials The year 1934 marked the
beginning of more rigorous standards for the formulas. Ralph Ojemann (1934)
did not invent a formula, but he did invent a method of assessing the difficulty of
materials for adult parent-education materials. His criterion was 16 passages of
about 500 words taken from magazines. He was the first to use adults to
establish the difficulty of his criterion. He assigned each passage the grade level
of adult readers who were able to answer at least one-half of the multiple-choice
questions about the passage.
Ojemann was then able to correlate six factors of vocabulary difficulty and eight
factors of composition and sentence structure with the difficulty of the criterion
passages. He found that the best vocabulary factor was the difficulty of words as
stated in the Thorndike word list.
Even more important was the emphasis that Ojemann put on the qualitative
factors such as abstractness. He recommended using his 16 passages for
comparing and judging the difficulty of other texts, a method that is now known
as scaling (See “Text leveling” below). Although he was not able to express the
qualitative variables in numeric terms, he succeeded in proving they could not be
Dale and Tyler: Adults of limited reading ability After working with Waples,
Ralph Tyler became interested in adults of limited reading ability. He joined
with Edgar Dale to publish (1934) their own readability formula and the first
study on adult readability formulas. The specific contribution of this study was
the use of materials specifically designed for adults of limited reading ability.
Their criterion for developing the formula was 74 selections on personal health
taken from magazines, newspapers, textbooks, and adaptations from children’s
health textbooks. They determined the difficulty of the passages with multiple-
choice questions based on the texts given to adults of limited reading ability.
The Principles of Readability
Copyright © 2004 William H. DuBay
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From the 29 factors that had been found significant for children’s
comprehension, they found ten that were significant for adults. They found that
three of these factors correlated so highly with the other factors that they alone
gave almost the same prediction as the combined ten. They were:
Number of different technical words.
Number of different hard non-technical words.
Number of indeterminate clauses.
They combined these three factors into a formula to predict the proportion of
adult readers of limited reading ability who would be able to understand the
material. The formula correlated .511 with difficulty as measured by multiple-
choice reading tests based on the 74 criterion selections.
The Ojemann and Dale-Tyler studies mark the beginning of work on adult
formulas that would continue unabated until the present time.
Lyman Bryson: Books for the average reader During the depression of the
1930’s, the government in the U.S. put enormous resources into adult education.
Bryson Lyman first became interested in non-fiction materials written for the
average adult reader while serving as a leader in adult-education meetings in
New York City. What he found was that what kept people from reading more
was not lack of intelligence, but the lack of reading skills, a direct result of
limited schooling.
He also found out there is a tendency to judge adults by the education their
children receive and to assume the great bulk of people have been through high
school. At that time, 40 to 50 million people had a 7th to 9th grade education and
reading ability.
Writers had assumed that readers had an equal education to their own or at least
an equal reading ability. Highly educated people failed to realize just how much
easier it is for them to read than it is for an average person. They found it
difficult to recognize difficult writing because they read so well themselves.
Although college and business courses had long promoted ideas expressed in a
direct and lucid style, Bryson found that simple and clear language was rare. He
said such language results from “a discipline and artistry which few people who
have ideas will take the trouble to achieve… If simple writing were easy, many
of our problems would have been solved long ago” (Klare and Buck, p. 58).
Bryson helped set up the Readability Laboratory of the Columbia University
Teachers College with Charles Beard and M. A. Cartwright. The purpose of the
laboratory was not to rewrite the classics or to help the beginning reader. The
purpose was to produce readable books on serious subjects for the average
Bryson understood that people with enough motivation and time could read
difficult material and improve their reading ability. Experience, however,
showed him that most people do not do that.
Perhaps Bryson’s greatest contribution was the influence he had on his two
students, Irving Lorge and Rudolf Flesch.
Gray and Leary: what makes a book readable William S. Gray and Bernice
Leary (1935) published a landmark work in reading research, What Makes a
Book Readable. Like Dale and Tyler’s work, it attempted to discover what
makes a book readable for adults of limited reading ability.
Their criterion included 48 selections of about 100 words each, half of them
fiction, taken from the books, magazines, and newspapers most widely read by
adults. They established the difficulty of these selections by a reading-
The Principles of Readability
Copyright © 2004 William H. DuBay
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comprehension test given to about 800 adults designed to test their ability to get
the main idea of the passage.
No subsequent work has examined readability so thoroughly or investigated so
many style elements or the relationships between them. The authors first
identified 228 elements that affect readability and grouped them under these four
1. Content
2. Style
3. Format
4. Features of Organization
The authors found that content, with a slight margin over style, was most
important. Third in importance was format, and almost equal to it, “features of
organization,” referring to the chapters, sections, headings, and paragraphs that
show the organization of ideas (See Figure 4).
Fig 4. The four major categories of readability (Gray and Leary, p. 31).
They found they could not measure content, format, or organization statistically,
though many would later try (See below, “The measurement of content”). While
not ignoring the other three causes, Gray and Leary concentrated on 80 variables
of style, 64 of which they could reliably count. They gave several tests to about a
thousand people. Each test included several passages and questions to show how
well the subjects understood them.
The Principles of Readability
Copyright © 2004 William H. DuBay
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Fig. 5. The four basic elements of reading ease.
Having a measure, now, of the difficulty of each passage, they were able to see
what style variables changed as the passage got harder. They used correlation
coefficients to show those relationship.
Of the 64 countable variables related to reading difficulty, those with
correlations of .35 or above were the following (p.115):
1. Average sentence length in words: -.52 (a negative correlation, that is,
the longer the sentence the more difficult it is).
2. Percentage of easy words: .52 (the larger the number of easy words the
easier the material).
3. Number of words not known to 90% of sixth-grade students: -.51
4. Number of “easy” words: .51
5. Number of different “hard” words: -.50
6. Minimum syllabic sentence length: -.49
7. Number of explicit sentences: .48
8. Number of first, second, and third-person pronouns: .48
9. Maximum syllabic sentence length, -.47
10. Average sentence length in syllables, -.47
11. Percentage of monosyllables: .43
12. Number of sentences per paragraph: .43
13. Percentage of different words not known to 90% of sixth-grade
students: -.40
14. Number of simple sentences: .39
15. Percentage of different words: -.38
16. Percentage of polysyllables: -.38
17. Number of prepositional phrases: -35
Although none of the variables studied had a higher correlation than .52, the
authors knew by combining variables, they could reach higher levels of
correlation. Because combining variables that were tightly related to each other
did not raise the correlation coefficient, they needed to find which elements were
highly predictive but not related to each other.
Gray and Leary used five of the above variables, numbers 1, 5, 8, 15, and 17, to
create a formula, which has a correlation of .645 with reading-difficulty scores.
An important characteristic of readability formulas is that one that uses more
variables may be only minutely more accurate but much more difficult to
measure and apply. Later formulas that use fewer variables may have higher
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 19
Gray and Leary’s work stimulated an enormous effort to find the perfect
formula, using different combinations of the style variables. In 1954, Klare and
Buck listed 25 formulas for children and another 14 for adult readers. By 1981,
Klare noted there were over 200 published formulas.
Research eventually established that the two variables commonly used in
readability formulas–a semantic (meaning) measure such as difficulty of
vocabulary and a syntactic (sentence structure) measure such as average sentence
length–are the best predictors of textual difficulty.
Some experts consider the number of morphemes for each 100 words to be a
major contributor to semantic (meaning) difficulty and the number of Yngve
word depths (branches) in each sentence to be a major contributor to syntactic
(sentence) difficulty. One study (Coleman 1971) showed that Flesch’s index of
syllables for each 100 words correlates .95 with morpheme counts. Another
study (Bormuth 1966) found that the number of words in each sentence
correlates .86 with counts of Yngve word depths. Measuring the average number
of syllables per word and the number of words in each sentence is a much easier
method and almost as accurate as measuring morphemes and word depths.
Formula limitations Readability researchers have long taken pains to
recommend that, because of their limitations, formulas are best used in
conjunction with other methods of grading and writing texts. Ojemann (1934)
warned that the formulas are not to be applied mechanically, a caution expressed
throughout readability literature. Other investigators concerned with the
difficulty and density of concepts were Morriss and Holversen (1938) and Dolch
(1939). E. Horn (1937) warned against the mechanical use of the word lists in
the re-writing of books for social studies.
George Klare and colleagues (1969) stated, “For these reasons, formula scores
are better thought of as rough guides than as highly accurate values. Used as
rough guides, however, scores derived from readability formulas provide quick,
easy help in the analysis and placement of educational material.”
Readability researchers such as Flesch (1949, 1964, 1979), Klare and Buck
(1954), Klare (1980), Gunning (1952), Dale (1967), Gilliland (1972), and Fry
(1988) wrote extensively on the other rhetorical factors that require attention
such as organization, content, coherence, and design. Using the formulas
creatively along with techniques of good writing results in greater
comprehension by an audience of a specified reading ability (Klare 1976, Chall
and Conard 1991).
Irving Lorge: Consolidating the research. Irving Lorge (1938) published The
Semantic Count of the 570 Commonest English Words, a frequency count of the
meaning of words rather than the words themselves. He later he was co-author
of E. L. Thorndike’s last book, The Teacher’s Word Book of 30,000 Words
Lorge was interested in psychological studies of language and human learning.
At Columbia, he came under the influence of Bryson at the Readability Lab. He
wanted a simple formula for predicting the difficulty of children’s books in terms
of grade scores. For the criterion for his own formula, Lorge (1939) used the 376
selections taken from the McCall-Crabbs Standard Test Lessons in Reading
(1926). He then standardized those passages on the basis of questions answered
by children in terms of the Thorndike-McCall Reading Scale. Using correlation
techniques, he was able to show that various combinations of factors gave
predictions of higher accuracy than the Gray-Leary formula.
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Copyright © 2004 William H. DuBay
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Though created for children’s reading, Lorge’s formula was soon widely used
for adult material as well. Where Gray and Leary’s formula had five elements,
Lorge’s had three, setting a trend that was to follow:
Average sentence length in words
Number of prepositional phrases per 100 words
Number of hard words not on the Dale list of 769 words.
Lorge’s use of the McCall-Crabbs Standard Test Lessons in Reading as a
criterion of difficulty greatly simplified the problem of matching readers to texts
(Klare, 1985). Although these passages were far from ideal, they had fewer faults
than other passages used as criteria. They remained the standard for readability
studies until the Bormuth studies in 1969.
The problem of communication to masses of people became especially obvious
during World War II. The government bureaus and the armed services needed
efficient ways of assessing the readability of their materials. Lorge’s formula was
one of the first, and it came into wide use (Klare and Buck, p. 59).
Rudolf Flesch and the art of plain writing The one perhaps most responsible
for publicizing the need for readability was Rudolf Flesch, a colleague of Lorge
at Columbia University. Besides working as a readability consultant, lecturer,
and teacher of writing, he published a number of studies and nearly 20 popular
books on English usage and readability. His best-selling books included The Art
of Plain Talk (1946), The Art of Readable Writing (1949), The Art of Clear
Thinking (1951), Why Johnny Can’t ReadAnd What You Can Do About It
(1955), The ABC of Style: A Guide to Plain English (1964), How to Write in
Plain English: A Book for Lawyers and Consumers (1979).
Flesch was born in Austria and got a degree in law from the University of
Vienna in 1933. He practiced law until 1938, when he came to the U.S. as a
refugee from the Nazis.
Fig. 6. Rudolf Flesch. The first
edition of
The Art of Plain Talk
1946 was a best seller. The
readability formulas it featured
started a revolution in journalism
and business communication.
Since his law degree was not recognized, he worked
several other jobs, one of them in the shipping
department of a New York book manufacturer.
In 1939, he received a refugee’s scholarship at
Columbia University. In 1940, he received a
bachelor’s degree with honors in library science. That
same year, he became an assistant to Lyman Bryson in
the Teachers’ College Readability Lab.
In 1942, Flesch received a master’s degree in adult
education. The next year, he received a Ph.D. in
educational research for his dissertation, “Marks of a
Readable Style” (1943). This paper set a course for his
career and that of readability.
In his dissertation, Flesch published his first
readability formula for measuring adult reading
material. One of the variables it used was affixes and
another was “personal references” such as personal
pronouns and names. Publishers quickly discovered
that Flesch’s formula could increase readership by 40
to 60 percent. Investigators in many fields of
communication began using it in their studies.
In 1948, Flesch published a second formula with two parts. The first part, the
Reading Ease formula, dropped the use of affixes and used only two variables,
the number of syllables and the number of sentences for each 100-word sample.
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Copyright © 2004 William H. DuBay
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It predicts reading ease on a scale from 1 to 100, with 30 being “very difficult”
and 70 being “easy.” Flesch (p. 225) wrote that a score of 100 indicates reading
matter understood by readers who have completed the fourth grade and are, in
the language of the U.S. Census barely “functionally literate.”
The second part of Flesch’s formula predicts human interest by counting the
number of personal words (such as pronouns and names) and personal sentences
(such as quotes, exclamations, and incomplete sentences).
The formula for the updated Flesch Reading Ease score is:
Score = 206.835 – (1.015 x ASL) – (84.6 x ASW)
Score = position on a scale of 0 (difficult) to 100 (easy), with 30 = very
difficult and 70 = suitable for adult audiences.
ASL = average sentence length (the number of words divided by the number
of sentences).
ASW = average number of syllables per word (the number of syllables
divided by the number of words).
This formula correlates .70 with the 1925 McCall-Crabbs reading tests and .64
with the 1950 version of the same tests.
In The Art of Readable Writing, Flesch (1949, p. 149), described his Reading
Ease scale in this way:
Ease Score
Estimated Reading
Estimated Percent
of U.S. Adults
0 to 30:
30 to 40:
50 to 60:
60 to 70:
70 to 80:
80 to 90:
90 to 100:
Very Difficult
Fairly Difficult
Fairly Easy
Very Easy
College graduate
13th to 16th grade
10th to 12th grade
8th and 9th grade
7th grade
6th grade
5th grade
Table 3. Flesch’s Reading Ease Scores
Flesch’s Reading Ease formula became the most widely used formula and one of
the most tested and reliable (Chall 1958, Klare 1963).
In an attempt to further simplify the Flesch Reading Ease formula, Farr, Jenkins,
and Paterson (1951) substituted the average number of one-syllable words per
hundred words for Flesh’s syllable count. The modified formula is:
New Reading Ease score = 1.599 nosw – 1.015 sl – 31.517
Where: nosw = number of one-syllable words per 100 words;
sl = average sentence length in words
This formula correlates better than .90 with the original Flesch Reading Ease
formula and .70 with 75% comprehension of 100-word samplings of the McCall-
Crabbs reading lessons. In 1976, a study commissioned by the U.S. Navy
modified the Reading Ease formula to produce a grade-level score, This popular
formula is known as the Flesch-Kincaid formula, the Flesch Grade-Scale formula
or the Kincaid formula (See “The Navy Readability Indexes” below).
In 1949, Flesch published the results of a 10-year study of the editorial content
of several magazines. He found that:
About 45% of the population can read The Saturday Evening Post.
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Copyright © 2004 William H. DuBay
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Nearly 50% of the population can read McCall’s, Ladies Home
Journal, and Woman’s Home Companion.
Slightly over 50% can read American Magazine.
80% of the population can read Modern Screen, Photoplay, and three
confession magazines.
Flesch (1949, pp. 149-150) compared the reading scores of popular magazines
with other variables:
Style Flesch
Length in
No. of Syll.
Per 100
Type of
Percent of
U.S. Adults
90 to 100 8 or less 123 or less Comics 4th grade 93
Easy 80 to 90 11 131 Pulp
5th grade 91
70 to 80 14 139 Slick
6th grade 88
Standard 60 to 70 17 147 Digests 7th or 8th
50 to 60 21 155 Quality Some high
Difficult 30 to 50 25 167 Academic High school
or some
0 to 30 29 or
192 or
Scientific College 4.5
Table 4. Flesch’s1949 analysis of the readability of adult reading materials.
Flesch’s work had an enormous impact on journalism. Like Robert Gunning,
who worked with the United Press, Flesch was a consultant with the Associated
Press. Together, they helped to bring down the reading grade level of front-page
stories from the 16th to the 11th grade, where they remain today.
Fig. 7. Edgar Dale, one of
the creators of
The Living
Word Vocabulary
stressed the importance
of vocabulary in assessing
The Dale and Chall Original Formula Edgar Dale, for
25 years a professor of education at Ohio State University,
was a respected authority on communications. He worked
his whole life to improve the readability of books,
pamphlets, and newsletters—the stuff of everyday reading.
Dale was one of the first critics of the Thorndike lists. He
claimed it failed to measure the familiarity of words
accurately. He subsequently developed new lists that were
later used in readability formulas.
One of these was a formula he developed with Jeanne
Chall, the founder and director for 20 years of the Harvard
Reading Laboratory. She had led the battle for teaching
early reading systematically with phonics. Her 1967 book
Learning to Read: The Great Debate, brought research to
the forefront of the debate. For many years, she also was
the reading consultant for TV’s Sesame Street and The
Electric Company.
The original Dale-Chall formula (1948) was developed for adults and children
above the 4th grade. They designed it to correct certain shortcomings in the
Flesch Reading Ease formula. It uses a sentence-length variable plus a
The Principles of Readability
Copyright © 2004 William H. DuBay
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percentage of “hard words”–words not found on the Dale-Chall “long list” of
3,000 easy words, 80 percent of which are known to fourth-grade readers.
To apply the formula:
1. Select 100-word samples throughout the text (for books, every tenth
page is recommended).
2. Compute the average sentence length in words.
3. Compute the percentage of words outside the Dale list of 3,000 words.
4. Compute this equation:
Score = .1579PDW + .0496ASL + 3.6365
Where: Raw score = reading grade of a reader who can answer one-half of
the test questions on a passage.
PDW= Percentage of Difficult Words (words not on the Dale-Chall
word list)
ASL = Average Sentence Length in words.
The Raw Score needs to be corrected at the higher grades. Dale and Chall
included with their formula the following chart for correcting the Raw Scores.
Raw Score Dale-Chall Score
4.9 and below Grade 4 and below
5.0 to 5.9 Grades 5-6
6.0 to 6.9 Grades 7-8
7.0 to 7.9 Grades 9-10
8.0 to 8.9 Grades 11-12
9.0 to 9.9 Grades 13-15 (college)
10 and above Grades 16 and above (college graduate)
Table 5. Dale-Call score-correction chart.
Of all the formulas produced in the early classic period, validations of this
formula have produced the most consistent, as well as some of the highest
correlations. It correlated .70 with the multiple-choice test scores on the McCall-
Crabbs reading lessons. You can find a computerized version of this original
formula online at:
Those interested in manually applying this formula can find the original 1948
Dale-Chall easy word list online at:
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Copyright © 2004 William H. DuBay
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Fig. 8. Robert Gunning.
His firm was the first
dedicated to improving
readability of journalism
and business writing.
Robert Gunning and the technique of clear writing
Robert Gunning was a graduate of Ohio State
University. In 1935, he entered the field of textbook
publishing. In the mid-1930s, educators were beginning
to see high school graduates who were not able to read.
Gunning realized that much of the reading problem was
a writing problem. He found that newspapers and
business were full of “fog” and unnecessary complexity.
Gunning was among the first to take the new readability
research into the workplace. In 1944, he founded the
first consulting firm specializing in readability. During
the next few years, he tested and worked with more than
60 large city daily newspapers and the popular
magazines, helping writers and editors write to their
In The Technique of Clear Writing, Gunning (1952) published a readability
formula developed for adults, the Fog Index, which became popular because of
its ease of use. It uses two variables, average sentence length and the number of
words with more than two syllables for each 100 words.
Grade Level = .4 (average sentence length + hard words)
Hard words = number of words of more than two syllables
Gunning developed his formula using a 90% correct-score with the McCall-
Crabbs reading tests. This gives the formula a higher grade criterion than other
formulas except for McLaughlin’s SMOG formula, which is based on a 100%
correct-answer criterion. The grade-level scores predicted by these two formulas
tend to be higher than other formulas.
Gunning found that popular magazines were consistent in their reading levels
over time. He published these correlations between reading levels of different
classes of magazines and their total circulation (p. 35). See Table 6.
Group Approx. Total
of Hard
Total Fog
Class Fewer than 1
20 10 30 12
News About 3 million 16 10 26 10
8 million 15 7 22 9
Slicks More than 10
15 5 20 8
Pulps More than 10
15 3 16 6
Table 6. Gunning’s analysis of the readability of adult reading materials.
The validation of the original Fog formula has never been published. According
to this author’s calculations, however, it correlates .93 with the normed reading
texts of Chall et al. (1996), a figure which may account for its popularity.
The Principles of Readability
Copyright © 2004 William H. DuBay
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Sumner, and Kearl (1958) recalculated the Fog formula using the McCall-Crabbs
reading lessons. The recalculated Fog formula, shown here, correlates .59 with
the reading passages.
Grade level = 3.0680 + .0877 (average sentence length) + .0984 (percentage
of monosyllables)
The publication of the Flesch, Dale-Chall, and Gunning formulas conveniently
marks the end of the first 30 years of classic readability studies. The authors of
these formulas brought the issue of readability to public attention. They
stimulated new consumer demands for documents in plain language. Finally,
they stimulated new studies, not only on how to improve the formulas, but also
on the other factors affecting reading success.
The New Readability Studies
The new readability was a period of consolidation and deeper study.
Investigators sought to learn more about how the formulas work and how to
improve them.
In the 1950s, several other developments accelerated the study of readability.
The challenges of Sputnik and the demands of new technologies created a need
for higher reading skills in all workers. While the older manufacturing industries
had little demand for advanced readers, new technologies required workers with
higher reading proficiency.
The New Readability studies were characterized by these features:
A community of scholars. The periodical summaries of the progress of
readability research (Klare 1952, 1963, 1974-75, 1984, Chall 1958,
and Chall and Dale 1995) revealed an exceptionally successful group of
scholars. They were often neglected by the others in the academe
because of their work on practical rather than theoretical problems.
They studied how and why the formulas work, how to improve them,
and what they tell us not only about reading, but also about writing.
The cloze test. The introduction of the cloze test by Wilson Taylor in
1953 opened the way for investigators to test the properties of texts and
readers with more accuracy and detail.
Reading ability, prior knowledge, interest, and motivation. A
number of studies looked at the manner in which these reader variables
affect readability.
Reading efficiency. While other studies looked at the effects of
readability on comprehension, these studies looked at the effects on
reading speed and persistence.
The measurement of content. The influence of cognitive psychology
and linguistics in the 1980s stimulated renewed studies of cognitive and
structural factors in the text and how they can be used to predict
Text leveling. Cognitive and linguistic theory revived interest in the
qualitative and subjective assessment of readability. With training,
leveling can be effective in assessing the elements of texts not
addressed by the formulas.
Producing and transforming text. Several studies examined the
effectiveness of using the formula variables to write and revise texts.
When writers attend to content, organization, and coherence, using the
The Principles of Readability
Copyright © 2004 William H. DuBay
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readability variables can be effective in producing and transforming a
text to a required reading level.
New readability formulas. Extensive studies of readability by John
Bormuth and others looked at the reliability of a wide range of
measurable text variables. They produced an empirical basis for
criterion scores and criterion texts for the development of new formulas
and reworking of old ones.
Formula discrepancy A look a the discrepancy between the results of
different formulas and how writers can benefit from it.
A Community of Scholars
Fig. 9. George Klare. After
serving as a navigator for th
U.S. Air Force in WWII (in
which he was shot down an
captured by the Germans),
Klare became a leading
figure in readability
Two notable features of readability research were a
community of scholars and a long research base. The
recognized bibliographer of that effort was George R.
Klare, now Distinguished Professor Emeritus of
Psychology and former Dean of the College of Arts and
Sciences at the Ohio University. Formerly the Dean of the
Department of Psychology, his field was psychological
statistics and testing as well as readability measurement.
Klare not only reviewed readability research (1963, 1974-
75, 1984), but he also directed and participated in
landmark studies and took the results of research to the
public. His reviews established the validity of the
formulas and their proper use not only in English, but also
in many other languages. Among Klare’s many important
publications were:
Know Your Reader: The Scientific Approach to
Readability, which he wrote with Byron Buck (1954).
The Measurement of Readability (1963).
“Assessing Readability in the Reading Research Quarterly (1974-75).
The Institute for Scientific Information recognized it as a Citation
Classic, one of the scientific works most frequently cited in other
studies—with well over 125 citations so far.
“A Second Look at the Validity of the Readability Formulas” in The
Journal of Reading Behavior (1976).
“Readable Technical Writing: Some Observations” in Technical
Communication (1977), which won “Best of Show” in the International
Conference of the STC in Dallas in 1978.
A Manual for Readable Writing (1975).
How to Write Readable English (1980).
“Readability” in Encyclopedia of Educational Research (1982).
“Readability” in TheHandbook of Reading Research (1984).
“Readable Computer Documentation” in the ACM Journal of Computer
Documentation (2000), which covered the latest research in readability.
Critics of the formulas (e.g., Redish and Selzer 1985) complained that the
readability formulas were developed for children and they never were never
formulated or tested with technical documents. The record shows, however, that
popular formulas such as the Flesch Reading Ease and the Kincaid formulas
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Copyright © 2004 William H. DuBay
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were developed mainly for adults and have been tested extensively on adults
with adult materials. For example, Klare (1952) tested the Lorge, Flesch Reading
Ease, and Dale-Chall formulas against the 16 standardized passages of the
Ojemann tests (1934) and the 48 passages of Gray and Leary (1935) tests, all
developed for adult readers.
As we will see, several extensive studies (Klare et al. 1955a, Klare et al. 1957,
Klare and Smart 1973, Caylor et al. 1973, Kincaid et al. 1975, Hooke et al.
1979) used materials developed for technical training and regulations in the
military to formulate and test several of today’s most popular formulas such as
the Flesch-Kincaid grade-level formula.
Perhaps Klare's most important studies were those confirming the effects of prior
knowledge, reading ability, interest, and motivation on adult reading (See
The Cloze Test
Wilson Taylor (1953) of the University of Illinois published “Cloze Procedure:
A New Tool for Measuring Readability.” Taylor cited several difficulties with
the classic readability formulas such as the Flesch and Dale-Chall. He noted, for
instance, that Gertrude Stein’s works measured much easier on the readability
scales than expected.
Taylor argued that words are not the best measure of difficulty but how they
relate to one another. He proposed using deletion tests called cloze tests for
measuring an individual’s understanding of a text. Cloze testing is based on the
theory that readers are better able to fill in the missing words as their reading
skills improve.
A cloze test uses a text with regularly deleted words (usually every fifth word)
and requires the subjects to fill in the blanks. The percentage of words correctly
entered is the cloze score. The lower the score, the more difficult the text.
Because even advanced readers cannot correctly complete more than 65% of the
deleted words correctly in a simple text, texts for assisted reading require a cloze
score of 35% or more. Texts for unassisted reading need a higher score. Cloze
scores line up with scores from multiple-choice tests in the following manner:
Purpose Cloze Multiple-Choice
50-60% 70-80%
assisted reading
35-50% 50-60%
Frustration level Below 35% Below 50%
Table 7. Comparison of cloze and multiple-choice scores.
For the origins of these scores, see “The Problem of Optimal Difficulty” below.
A cloze test uses a text with selected words deleted and replaced with underlines
of the same length. Having at least 50 blanks in the reading selection increases
the reliability of the test.
To score a cloze test, use the percentage of all the words that are correctly
entered, that is, the right words in the right form (no synonyms), number, person,
tense, voice, and mode. Do not count spelling.
It greatly increases the accuracy of the test to test all the words by using different
versions of the text. If you delete every 5th word, there are five possible versions,
each one with a different first deleted word. Divide the subjects into as many
groups as you have versions and give each group a different version.
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Copyright © 2004 William H. DuBay
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Here is a sample cloze test:
The potential for two-way _______ is very strong on ________ Web. As a
result, ________ companies are focused on ________ Web’s marketing
potential. From ________ marketing point of view, ________ virtual worlds
can attract ________curious Web explorers, and ________database engines
can measure ________ track a visitor’s every ________.
See the answers at the end of this article. Note that the standard cloze test does
not provide a list of the correct words to choose from as some online cloze
programs do.
Cloze testing became the object of intensive research, with over a thousand
studies published (Klare 1982). It quickly became popular as a research tool, and
tended to complement not the formulas as expected but conventional reading
tests. Unlike multiple-choice tests, cloze tests can provide suggestive information
about individual sentences, clauses, phrases, and words. Cloze tests are suitable
for intermediate and advanced readers. Cloze testing opened the way for much
more intensive studies of the readability formulas, beginning with Bormuth in
1966 (see below).
Reading Ability, Prior Knowledge, Interest, and Motivation
The interest factors affecting the readability of children’s literature was taken up
by Gates (1930) and Zeller (1941). One of the interest factors that Gates
mentioned for children was reading ease. Flesch’s early formula for adults
(1949) included interest factors for measuring readability. The new research
would establish that, along with vocabulary and sentence structure, the reader’s
reading ability, prior knowledge and motivation are powerful contributors to text
Prior knowledge and retention A series of studies in the military (Klare et al.
1955a) examined how prior knowledge as well as the text variables affect the
retention and the acceptability of technical documents.
The studies were conducted at Sampson Air Force Base in New York and
Chanute Air Force Base in Illinois using 989 male Air Force enlistees in training
with different versions of the same texts. They used the Flesch Reading Ease,
Dale-Chall, and the Flesch Level-of-Abstraction formulas to rate the texts as
Easy (grade 7), Present (12th grade), and Hard (16th grade).
While simplifying documents and changing the style, they retained all technical
terms and used technical experts to assure that they did not change the content.
This study found the more readable versions resulted in:
Greater and more complete retention.
Greater amount read in a given time.
Greater acceptability (preference).
The study found that, “…while style difficulty appears to affect immediate
retention of subjects who are naïve regarding material, subjects who have
considerable knowledge of the material may profit little if any from an easier
style of material” (p. 294).
Duffy (1985) criticizes the results of this study. He states that the 8% percent
improvement in comprehension, achieved by dropping the reading level of the
texts eight grades (from the 16th+ grade to the 7th-8th grades—1% improvement
for each grade dropped) is not large enough to justify the effort required.
Duffy underestimates the difficulty of demonstrating the comprehension gained
by changing any textual variable while carefully controlling the other variables
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Copyright © 2004 William H. DuBay
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done in the study. Most researchers are very happy to get any non-chance
improvements in comprehension, the holy grail of reading research.
The difficulty arises from the complexity of reading comprehension and the
means we have of testing it, which are all indirect. Researchers, for example, are
not sure exactly what the results of reading tests are telling them. Do they reveal
comprehension of the text or other artifacts such as prior knowledge, memory, or
the difficulty of the questions?
Studies of the effects of textual variables and writing strategies on
comprehension are very often inconsistent, inconclusive, or non-existent.
Examples include: the use of illustrations (Halbert 1944, Vernon 1946, Omaggio
1979; Felker et al., 1981), schemas (Rumelhart 1984), structural cues
(Spyridakis, 1989, 1989a), highlighting (Klare et al. 1955b, Felker et al.),
paragraph length (Markel, et al., 1992), typographic format (Klare 1957), syntax
simplification (Ulijn and Strother 1990), prior knowledge (Richards 1984),
nominalizations, diagrams, parallelism, white space, line graphs, and justified
margins (Felker et al,), “whiz deletions” (Huckin et al. 1991), writer guidelines
(McLean 1985), and coherence and cohesion (Freebody and Anderson 1983,
Halliday and Hasan 1976).
No one would say that any of these items are not helpful or do not affect
comprehension. These studies show, however, how difficult it is to detect and
measure the effect of a reading variable on comprehension. Any significant gain
in comprehension, even a small one, can be important over time and suggests
further study. In this regard, studies confirm that the formulas do very well. See
“Producing and transforming texts” below.
Career preferences, aptitudes, and test scores. A further investigation by the
same authors (Klare et al. 1955c) looked into the effect of career aptitude and
preferences on immediate retention. As expected, the subjects with higher degree
of mechanical and clerical aptitude showed consistently higher retention on test
scores. There were no significant relationships, however, between career
preferences and retention.
Interest, Prior Knowledge, Readability and Comprehension A study (Klare
1976) of the experiments on the effects of using formulas to revise texts showed
how different levels of motivation and reading ability can skew the results. It
also indicated that the readability of a text is more important when interest is low
than when it is high. The study by Fass and Schumacher (1978) supports this
Woern (1977) later showed that prior knowledge and beliefs about the world
affected comprehension significantly. Pearson, Hanson, and Gordon (1979)
discovered significant effects of prior knowledge on the comprehension of
children reading about spiders. Spilich, Vesonder, Chiesi, and Voss (1979)
found that subjects having more knowledge about baseball remembered more
information about a baseball episode. Chiesi, Spilich, and Voss (1979) found
that high-knowledge subjects had better recognition, recall, and anticipation of
goal outcomes than did low-knowledge subjects.
Entin and Klare (1985) took up the interaction between the readability of the text
and the prior knowledge and interest of the readers. The study used 66 students
enrolled in introductory psychology courses at Ohio University. They were first
tested with the Nelson-Denny Reading Test to determine reading skills. They
were then given a questionnaire on their interest in selected topics and a
questionnaire on their prior knowledge of the terminology used in the test
passages. For test passages, they used 12 selected passages from the World Book
Encyclopedia, six high-interest passages, and six low-interest ones. The passages
The Principles of Readability
Copyright © 2004 William H. DuBay
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were re-written and normed by judges for content at the 12th and 16th-grade
levels, resulting in 24 passages for the experiment. Then, two cloze tests were
made of each passage, resulting in 48 test passages
This study confirmed that easier readability of a text has more benefits for those
of less knowledge and interest than those of more. Advanced knowledge of a
subject can “drown out” the effects of an otherwise difficult text.
This study also suggested that when reader interest is high, comprehension is not
improved by writing the material below, rather than at, the grade level of the
readers. When interest is low, however, comprehension is improved by writing
the materials below, rather than at, the reading level of the readers.
Comprehension was improved when the materials are written at the reading
levels of all readers rather than above those levels.
Reading Performance
While early studies used reader comprehension as a measure of readability, new
studies were looking at other performance measures such as:
Reading persistence (or perseverance)
Reading efficiency
Readability and reader persistence Several studies in the field of journalism
found a significant relationship between reader persistence and readability. Some
used split runs of newspapers to see the effects of improved readability on wide
Donald Murphy (1947), the editor of Wallace’s Farmer, used a split run with an
article written at the 9th-grade level on one run and on at the 6th-grade level on
the other run. He found that increasing readability increased readership up of the
article 18 percent. In a second test, he took great care not to change anything
except readability, keeping headlines, illustrations, subject matter and the
position the same. He found readership increases of 45% for an article on nylon
and 60% for an article on corn.
Wilbur Schramm (1947) showed that a readable style contributes to the readers’
perseverance, also called depth or persistence, the tendency to keep reading the
Charles E. Swanson (1948) showed that better readability increases reading
perseverance as much as 80 percent. He developed an easy version of a story
with 131 syllables per 100 words and a hard version with 173 syllables and
distributed each to 125 families. A survey of readers taken 30 hours after
distribution showed a gain in the easier version over the hard version of 93% of
total paragraphs read, 83% in mean number of paragraphs read, and 82% in the
number of correspondents reading every paragraph.
Bernard Feld (1948) grouped 101 stories from the Birmingham News into those
with high Flesch scores, requiring 9th-grade education or more and those with
low scores, requiring less than 9th-grade education. He found readership
differences of 20 to 75 percent favoring the low-score versions. Feld’s findings
indicated that even a small actual percentage gain for a large-circulation paper
greatly increased the number of readers.
Reading efficiency Klare, Shuford, and Nichols (1957) followed up these
studies with a study of the reading efficiency and retention of 120 male aviators
in a mechanics course at Chanute Air Force Base in Illinois. They used two
versions of technical training materials, hard (13th-15th grade) and easy (7th-8th
The Principles of Readability
Copyright © 2004 William H. DuBay
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They measured reading efficiency with an eye-movement camera with which
they could determine the number of words read per second and the number of
words read per fixation. A strong “set-to-learn” was stimulated by allowing the
subjects to re-read the text and giving them a pre-test before the experimental
The study showed that the easy text significantly improved both reading
efficiency and retention. The results also indicated that a strong “set to learn”
improved scores.
Hardyck and Petrinovich (1970) showed the connection between readability and
both comprehension and muscle activity in the oral area (subvocalization).
Rothkopf (1977) showed the connection between readability and how many
words a typist continues to type after the copy page is covered (functional
Readability and course completion Publishers of correspondence courses are
understandably concerned when large numbers of students do not complete the
courses. They often suspect the materials are too difficult for the students.
Working with Kim Smart of the U. S. Armed Forces Institute, Klare (1973)
applied the Flesch Reading Ease formula to thirty sets of printed correspondence
courses used by the military.
They found that two of the high school courses and five of the college courses
were too difficult for readers of average or below average reading skill.
They then compared their reading analysis to the completion records of the 17
courses that had been in use over two years. They found a Spearman rank-order
correlation of .87 between the readability score and the probability of students
completing the course. There was a Pearson product-moment correlation of .76.
These results showed the importance of readability for unassisted reading where
pressure to complete a course of study is low and competition from distractions
is high.
The Measurement of Content
For hundreds of years, writers and teachers have used and taught the cognitive
and structural factors in text such as organization and coherence. Researchers in
readability also addressed the effects of these factors on comprehension:
Image words, abstraction, predication, direct and indirect discourse,
types of narration, and types of sentences, phrases, and clauses (Gray
and Leary 1935).
Difficult concepts (Morriss and Holverson 1938, Chall 1958).
Idea density (Dolch 1939).
Human interest (Flesch 1949, Gunning 1952)
Organization (Gunning 1952, Klare and Buck 1954, Chall 1958).
Nominalization (Coleman and Blumenfeld 1963; Coleman, 1964)
Active and passive voice (Gough 1965, Coleman 1966, Clark and
Haviland 1977, Hornby 1974).
Embeddedness (Coleman 1966).
The cognitive theorists and linguists, beginning in the 1970s, promoted the idea
that reading was largely an act of thinking. Among the ideas they promoted
1. Meaning is not in the words on the page. The reader constructs
meaning by making inferences and interpretations.
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Copyright © 2004 William H. DuBay
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2. Information is stored in long-term memory in organized "knowledge
structures." The essence of learning is linking new information to prior
knowledge about the topic, the text structure or genre, and strategies
for learning.
3. A reader constructs meaning using metacognition, the ability to think
about and control the learning process (i.e., to plan, monitor
comprehension, and revise the use of strategies and comprehension);
and attribution, beliefs about the relationship among performance,
effort, and responsibility (Knuth and Jones 1991).
The cognitive theorists, aware of the limitations of the readability formulas, set
about to supplement them with ways to measure the content, organization, and
coherence of the text. Their studies reinforced the importance of these variables
for comprehension. They did not, however, come up with any practical method
for measuring or adjusting them for different levels of readers.
The following sections summarize a few of these efforts.
Walter Kintsch and coherence Beginning in 1977, Walter Kintsch and his
associates studied the cognitive and structural issues of readability. Kintsch
proposed to measure readability by measuring the number of propositions in a
text. A proposition consists of a predicate and one or more arguments. An
argument can be a concept or another argument. A concept is the abstract idea
conveyed by a word or phrase.
In the early part of his work, Kintsch (Kintsch and Vipond 1979) was quite
critical of the readability formulas. He said they are not based on modern
linguistic theory and they overlook the interaction between the reader and the
Over four years, however, he and his associates revised this position. He
eventually admitted that “these formulas are correlated with the conceptual
properties of text” and that vocabulary and sentence length are the strongest
predictors of difficulty (Kintsch and Miller 1981, p. 222).
While Kintsch and his colleagues did not come up with any easily used formula,
they did contributed to our understanding of readability, including the central
role of coherence in a text. Kintsch found out that lack of coherence affects
lower-grade readers much more than upper-grade ones. The upper-grade readers,
in fact, feel challenged to reorganize the text themselves. They may require more
opportunities for solving problems, while lower-grade readers require more
carefully organized texts.
The Air Force transformational formula. Perhaps the most ambitious attempt
to quantify the variables of the cognitive theorists and put them in a formula was
the project of Williams, Siegel, Burkett, and Groff (1977). Working for the Air
Force Human Resources Laboratory, they examined new variables, produced a
new formula, and presented supporting data. The variables they included were:
Four psycholinguistic variables such as Yngve word depths,
transformational complexity, center embedding, and right branching.
Four Structure of Intellect variables including cognition of semantic
units, memory for semantic units, evaluation of symbolic implications,
and divergent production of semantic units.
For a criterion, they used cloze scores on 14 passages of about 600 words each
taken from the Air Force career-development course. They deleted each tenth
word in the cloze test and used only one version out of a possible ten on 51 Air-
Force subjects. The computerized formula produced a correlation of 0.601 with
text difficulty.
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Copyright © 2004 William H. DuBay
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Susan Kemper and the reader’s mental load Following Kintsch, Susan
Kemper (1983) sought to explain comprehension in terms of underling cognitive
processes. She developed a formula designed to measure the “inference load”
based on three kinds of causal links:
Physical states
Stated mental states
Inferred mental states
The Kemper formula measures the density of the propositions and embedded
clauses. It takes considerable time and effort in comparison to the readability
formulas. It has a correlation of .63 with the McCall-Crabbs tests (the original
Dale-Chall formula has a correlation of .64).
Kemper (p. 399) commented: “..sentence length and word familiarity do
contribute to the comprehension of these passages…. These two different
approaches to measuring the grade level difficulty of texts are equivalent in
predictive power.”
Kemper admitted that her formula, like all readability formulas, is better at
predicting problems than fixing them. For writing, both formulas are best used as
a general guide.
Bonnie Meyer and organization Bonnie Meyer and others worked on using
the organization of larger units of texts as a possible measurement of readability.
She claimed that a text that follows a topical plan is more efficient (saves effort)
and more effective (gets more results). She wrote:
That is, people remember more and read faster information which is
logically organized with a topical plan than they do when the same
information is presented in a disorganized, random fashion…. Thus
the plan of discourse can be considered apart from content, and
deserves separate consideration from researchers, as from those who
are planning a composition (Meyer 1982, p. 38).
Among Meyer’s observations are the following:
A visible plan for presenting content plays a key role in assessing the
difficulty of a text.
A plan incorporates a hierarchy showing the dependencies of the facts
to one another:
The antecedent/consequent plan shows causal relationships in
“if/then” logic.
The comparison plan presents two opposing views that give weight to
both sides.
The adversative plan clearly favors one side over the other (political
The description plan describes the component parts of an item
(newspaper articles). This plan is the least effective for remembering
and recall.
The response plan gives answers to remarks, questions, and problems
(science articles).
The time-order plan relates events chronologically (history texts).
Better readers tend to share the same plan as authors of the material they are
reading. Readers who use a different plan other than the authors may be at a
The Principles of Readability
Copyright © 2004 William H. DuBay
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There are two types of highlighting for showing the relationships between items:
Subordination, used to connect the main idea with supporting text as
in a hierarchical structure.
Signaling, explicit markers to clarify relationships such as:
“On the one hand…On the other hand…
“Three things have to be stressed here.”
“Thus,” “consequently,” and “therefore”
“Nevertheless,” “all the same,” “although,” “but,” and “however”
Signaling can also clarify how larger blocks of content are related, for example:
“For example,” “For further details,” “summary,” “abstract,” “conclusion,” and
“preview.” For more on signaling, see the studies by Jan Spyridakis (1989,
Besides reducing the difficulty of the text, Meyer wrote that strategy training can
also help older adults deal with the difficulties they encounter in reading.
Bonnie Armbruster and textual coherence Also concerned with larger units
of text, Bonnie Armbruster (1984) found that the most important feature for
learning and comprehension is textual coherence, which comes in two types:
Global coherence, which integrates high-level ideas across an entire
section, chapter, or book.
Local coherence, which uses connectives to link ideas within and
between sentences.
Armbruster found that recalling stories from memory is superior when the
structure of the story is clear. She also noted the close relationship between
global content and organization. Content is an aspect of structure, and
organization is the supreme source of comprehension difficulty.
For local coherence, Armbruster stressed the highlighting that carries meanings
from one phrase, clause, or sentence to another:
Pronoun references to previous nouns
Substitutions or replacements for a previously used phrase or clause
(sometimes called “resumptive modifiers”), for example: “These results
[previously listed] suggest that…”
Finally, Armbruster supported Kintsch’s finding that coherence and structure are
more important for younger readers than older ones, simply because they have
less language and experience.
Calfee, Curley, and the familiar outline R.C. Calfee and R. Curley (1984)
built on the work of Bonnie Meyer. They stressed making the structure of the
text clear to upper-grade readers. The content can be simple, but an unfamiliar
underlying structure can make the text unnecessarily difficult.
They proposed that the teacher, researcher, and student all need to reach a
mutual understanding of the type of outline being used for the text under
Most students are familiar with the narrative structure, but not with other forms.
Calfee and Curley present a graduated curriculum that enables students to
progress from simpler structures to ones that are more difficult:
1. Narrative—fictional and factual
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2. Concrete process—descriptive and prescriptive
3. Description—fictional, factual particular, and factual general
4. Concrete topical exposition
5. Line of reasoning—rational, narrative, physical and relational cause-
6. Argument—dialogue, theories and support, reflective essay
7. Abstract exposition
The lessons of content, organization, and coherence Organization and
coherence highlight the relationships between words, sentences, paragraphs, and
larger sections of text. They enable readers to fit new items of information into
their own cognitive systems of organization.
The cognitive studies of readability also showed other problems that texts can
reveal or create, such as:
Unfamiliar life experiences and background
The need for time to digest illustrations and new material
The need for multiple treatments of difficult material
The need for learning aids to overcome textual difficulty
The need for learning aids to help readers of different levels of skill.
Generally, however, the cognitive researchers failed to translate their theories
into practical and objective methods for adjusting the difficulty of texts for
different levels of reading skill.
Critics of the formulas (e.g., Manzo 1970, Bruce et al. 1981, Selzer 1981,
Redish and Selzer 1985, Schriver 2000) rightly claim that the formulas use only
“surface features” of text and ignore other features like content and organization.
The research shows, however, that these surface features—the readability
variables—with all their limitations have remained the best predictors of text
difficulty as measured by comprehension tests (Hunt 1965, Bormuth 1966,
Maxwell 1978, Coupland 1978, Kintsch and Miller 1981, Chall 1984, Klare
1984, Davison 1984 and 1986, Carver 1990, Chall and Conard 1991, Chall and
Dale 1995).
Text Leveling
An important byproduct of the cognitive and linguistic emphasis was the
renewed interest in text leveling. This involves a subjective analysis of reading
level that examines vocabulary, format, content, length, illustrations, repetition
of words, and curriculum. Text leveling is perhaps the oldest method of grading
a text. The McGuffey readers were graded by leveling, and their success is an
indication of its validity.
Leveling recently became popular largely due to the work of the New Zealand
Department of Education. In the U.S., Marie Clay’s (1991) Reading Recovery
system uses leveling in tutoring of children with reading problems. In this
system, teachers use leveling to find books with closely spaced difficulty levels,
particularly at the first-and second-grade levels. Most traditional readability
formulas are not particularly sensitive at those levels (Fountas and Pinnell,
For that same reason, readability experts have long encouraged the use of
subjective leveling along with the readability formulas. Leveling can spot the
items that the formulas do not measure (Klare 1963, pp. 137-144; Chall et al.
1996; Fry 2002).
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R. P. Carver (1975-1976) introduced a method of using qualified raters to
assess the difficulty of texts. Raters become qualified when accurately judging
the difficulty of five passages using his “Rauding Scale,” consisting of six
passages representing grades 2, 5, 8, 11, 14, and 17. Carver claimed his method
was slightly more accurate than the Dale and Chall and Flesch Reading Ease
formulas and provides grade-level scores through grade 18.
H. Singer (1975) created a method called SEER, “Singer Eyeball Estimate of
Readability.” It involves the use of one or two accurate SEER judges matching a
sample of text against one of two scales, each consisting of eight rated passages.
Singer claims his method is as accurate as the Fry graph.
The problem, of course, is that it takes some effort to learn how to do leveling
accurately. Advanced readers often fail to recognize how difficult texts can be
for others. Leveling also becomes more effective and accurate as the number of
experienced judges increases (Klare, 1984).
Jeanne Chall and her associates (1996) published Qualitative Assessment of Text
Difficulty, A Practical Guide for Teachers and Writers. It uses graded passages,
called “scales,” from published works along with layouts and illustrations for
leveling of texts. You can assess the readability of your own documents by
comparing them to these passages and using the worksheet in the book. The 52
passages are arranged by grade level and by the following types of text:
Popular fiction
Life sciences
Physical sciences
Narrative social studies
Expository social studies
The scale passages were selected on the basis of the following grade-related
requirements for the reader:
1. Knowledge of vocabulary
2. Familiarity with sentence structure
3. Subject-related and cultural knowledge
4. Technical knowledge
5. Density of ideas
6. Level of reasoning
The selections were then tested by:
1. Evaluation by several groups of teachers and administrators
2. Evaluation by students of corresponding grades
3. Cloze testing of students of corresponding grades
4. Readability formulas (Dale-Chall and Spache)
The book also describes at length the various characteristics of each type of text
that can contribute to difficulty. An added section features samples of the design
and illustrations of books appropriate for the first four grades.
The following are three samples of the scales taken from the book.
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Copyright © 2004 William H. DuBay
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Reading Level 3
The stars, like the sun, are always in the sky, and they are always
shining. In the daytime the sky is so bright that the stars do not show.
But when the sky darkens, there they are.
What are the stars, you wonder, and how do they twinkle?
Stars are huge balls of hot, hot gas. They are like the sun but they look
small because they are much, much farther away. They are trillions and
trillions of miles away, shining in black space, high above the air.
Space is empty and does not move. Stars do not twinkle there, but
twinkling begins when starlight hits the air. The air moves and tosses
the light around.
—From The Starry Sky: An Outdoor Science Book (Wyler 1989, pp. 15-
Reading Level 5-6
Black holes are probably the weirdest objects in space. They are
created during a supernova explosion. If the collapsing core of the
exploding star is large enough—more than four times the mass of our
sun—it does not stop compressing when it gets as small as a neutron
star. The matter crushes itself out of existence. All that remains is the
gravity field—a black hole. The object is gone. Anything that comes
close to it is swallowed up. Even a beam of light cannot escape.
Like vacuum cleaners in space, black holes suck up everything around
them. But their reach is short. A black hole would have to be closer
than one light-year to have even a small effect on the orbits of the
planets in our solar system. A catastrophe such as the swallowing of
the Earth or the sun is strictly science fiction.
—From Exploring the Sky (Dickinson 1987, p. 42)
Reading Level 7-8
As we have seen, a neutron star would be small and dense. It should
also be rotating rapidly. All stars rotate, but most of them do so
leisurely. For example, our Sun takes nearly one month to rotate
around its axis. A collapsing star speeds up as its size shrinks, just as
an ice-skater during a pirouette speeds up when she pulls in her arms.
This phenomenon is a direct consequence of a law of physics known as
the conservation of angular momentum, which holds that the total
amount of angular momentum in a system holds constant. An ordinary
star rotating once a month would be spinning faster than once a second
if compressed to the size of a neutron star.
In addition to having rapid rotation, we expect a neutron star to have an
intense magnetic field. It is probably safe to say that every star has a
magnetic field of some strength.
—From Discovering the Universe (Faufmann 1990, p. 290)
Producing and Transforming Text
While the formulas were originally created to help educators select texts for
different audiences, writers also use the formula variables to produce texts and
transform (re-write) them into simpler versions. The evidence on how effective
this is has been mixed. As both the supporters of the formulas and their critics
have warned, if you just chop up sentences and use shorter words, the results are
not likely to improve comprehension. You have to look at the many other factors
that affect reading at the level for which you are writing.
Early evidence on the effects of using the formula variables to transform text was
negative. Klare (1963) reported that, of the six readability studies involving the
controlled manipulation of words or sentences, only one had a positive effect,
and this involved simplifying vocabulary.
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In a later study, Klare (1976) took a careful look at 36 studies that examined the
effects on comprehension of using the readability formula variables in re-writing
texts. He grouped them by their results:
19 studies had positive results (readability variables had a significant
effect on comprehension and/or retention)
6 studies had mixed results
11 studies had negative results (no measurable effect).
In seeking the reasons for the differences, Klare looked carefully at 28
situational factors in which each experiment was conducted. The situational
factors fell into these groups:
The readability and content of the material.
The competence and motivation of the subjects.
The instructions given the subjects during the experiment.
The details of the test situation.
Klare found that differences in readability were often overridden by other factors
in the test situation such as:
The instruction given to the subjects of the test.
The presence of threats or rewards.
The time allowed for reading and testing.
The presence or absence of the text during the test.
Klare wrote that the performance of the subject in such tests is a function not
only of the difficulty of the material, but also in critical degrees, a function of the
test situation (time, place, etc.), the content of the material and the competence
and motivation of the reader. Scores will be better, for instance, when the readers
love the subject matter or if they are highly motivated (e.g., paid).
Klare concluded that in the studies that showed increased comprehension,
transforming text requires attending to other problems besides word and
sentence length. “The best assumption, it seems to me,” he wrote, “is that the
research workers, probably with considerable effort, managed to change basic
underlying causes of difficulty in producing readable versions” (p. 148). Klare
then listed the following word-and-sentence variables that affected
Word characteristics:
1. Proportion of content (functional) words.
2. Frequency, familiarity, and length of content words.
3. Concreteness or abstractness.
4. Association value.
5. Active vs. nominalized verb constructions.
Sentence characteristics:
1. Length (esp. clause length).
2. Active vs. passive.
3. Affirmative vs. negative.
4. Embedded vs. non-embedded.
5. Low depth vs. high depth (branches).
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Since Klare’s 1976 study, there have been other studies showing the positive
effects of using formula variables to improve comprehension (Ewing 1976,
Green 1979, C. C. Swanson 1979).
In the many studies of before-and-after revision of the text, a negative result does
not prove that there is no improvement in comprehension. They show instead
that improvement has not been detected. There is a saying in statistics that you
cannot prove a negative.
Studies reporting a negative result may result from failing to control the reading
ability, prior knowledge, interest, and motivation of the subjects. They can also
result from failing to control elements of the text such as organization,
coherence, and design. The great difficulty of properly conducting such an
experiment is seen in following two studies.
The Duffy and Kabance study Critics worry that technical communicators can
too easily misuse the formulas, making documents more difficult, not less
(Charrow 1977, Kern, 1979, Selzer 1981, Lange 1982, Duffy 1985, Redish and
Selzer 1985, Connaster 1999, Redish 2000, Schriver 2000). These writers offer
little or no evidence of such misuse, however, widespread or otherwise. If
unscrupulous or careless writers choose to cheat by “writing to the formula” and
not attending to other textual issues, careful editors and reviewers easily spot the
misuse. The study by Thomas Duffy and Paula Kabance (1981) is a case in
point. Because formula critics (e.g., Redish and Selzer 1985; Redish 2000) often
refer to this study, it deserves some attention.
The Duffy and Kabance study consisted of four experiments that examined the
effects of changing only word and sentence length on comprehension. It used a
“reading to do” task and a “reading to learn” task. The study used four versions
of the text:
1. The original version (a narrative or expository passage from the 1973
Nelson-Denny reading tests).
2. One with vocabulary that they simplified using The Living Word
3. One with only simplified and shortened sentences.
4. One with both vocabulary and sentences simplified.
The effect was a 6-grade drop in reading level of the changed passages from the
11th to the 5th grade.
Following Klare’s research protocols (1976), they attempted to maximize the
readability effects by using readers who were low motivated, unfamiliar with the
topic, and widely varying in reading skills.
Using the Nelson-Denny reading tests, they tested the reading ability of the
1,169 subjects, male Navy trainees between 17 and 20 years old, of which 80%
were high-school graduates. They divided them into two groups, one with a
median reading grade of 8.7 and the other 10.3. The experiments took place in
groups of 40 to 70.
In the first two experiments, they simulated a “reading-to-do” situation. In the
first experiment, they first showed the questions, then had the subjects read the
text. After that, they were shown the questions again, which they answered. In
the second experiment, they limited the reading time but let the subjects have
access to the text while answering the questions. The third experiment was a
standard cloze test. The fourth experiment was a standard multiple-choice test
with the subjects first reading the text and then answering the questions without
the text.
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Copyright © 2004 William H. DuBay
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The first three experiments showed no significant improvements. The fourth
experiment resulted in significant improvement but only with the low-ability
group using the changed-vocabulary text, an improvement of 13 percent. The
authors concluded that simplifying the text made no difference to the advanced
readers. This is not a surprising result, when we consider the reading ability of
the advance group was at grade 10.3 while the difficult text was at 11th grade.
The vocabulary variable is significant for the low-ability group, they stated, but
only in reading-to-learn tasks but not reading-to-do tasks, where memory is less
important. This correlation was also suggested by Fass and Schumacher (1978).
Duffy and Kabance concluded that the increased readability is not required for
technical documents, in which the emphasis is on “reading to do” and memory is
not required.
This is sometimes true. At other times, serious errors have taken place because
of memory failure. Many, if not most, technical tasks involve learning a skill that
can be repeated, as Redish (1988) emphasizes. Besides reading-to-learn and
reading-to-do tasks, she writes, many technical tasks require “reading to learn to
do.” Technical texts may require more memory than do most other kinds of
literature such as magazines, newspapers, or fiction.
When we look at the methods of these experiments, difficulties appear that
explain their inconsistent results. In their report, Duffy and Kabance provide four
sample passages used in the study. The re-written passages appear disjointed and
stilted, not what one would expect of a 5th-grade text (See Fig. 11). If these
studies are representative of the other passages, we must assume that judges were
not used to control for the coherence and content of the text.
This was the also the conclusion of Leslie Olsen and Rod Johnson (1989), who
wrote: “In their study, Duffy and Kabance were trying to directly manipulate the
understanding of the words and the syntax of the sentences. However, it seemed
to us that they were also unintentionally altering other aspects of the text—in
particular, the cohesive structures of the text.”
In their paper, Olsen and Johnson defined “sensed cohesion” as the strength of
the textual topicality and the sense of givenness. The strength of textual
topicality is related to the persistence of what the text is about. The sense of
givenness is the recognition that the reader has seen a particular noun phrase
In analyzing the passages of the Duffy and Kabance study, Olsen and Johnson
found that long sentences were broken up into short sentences. In the process,
they introduced new subjects. The original focus on the Spaniards was lost,
making it difficult to know what the text is about. They analyzed the
cohesiveness of the text and concluded, “the intended and the unintended effects
of the revisions cancelled one another out,” bringing the results of the study into
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Copyright © 2004 William H. DuBay
Page 41
Original (11th Grade)
The night was cloudy, and a drizzling rain, which fell
without intermission, added to the obscurity. Steadily,
and as noiselessly as possible, the Spaniards made their
way along the main street, which had so lately
resounded to the tumult of battle. All was now hushed
in silence; they were only reminded of the past by the
occasional presence of some solitary corpse, or a dark
heap of the slain, which too plainly told where the strife
had been the hottest. As they passed along the lanes and
alleys which opened into the great street, they easily
fancied they discerned the shadowy forms of their foe
lurking in ambush, ready to spring upon them. But it
was only fancy; they city slept undisturbed even by the
prolonged echoes of the tramp of the horses, and the
hoarse rumbling of the artillery and baggage trains. At
length, a lighter space beyond the dusky line of
buildings showed the van of the army that it was
emerging on an open causeway. They might well have
congratulated themselves on having thus escaped the
dangers of an assault in the city itself, and that a brief
time would place them in comparative safety on the
opposite shore.
Sentences and Vocabulary Revised (5th Grade)
The night was cloudy. A sprinkling rain added to the
darkness. It fell without a break. The Spaniards made
their way along the main street. They moved without
stopping and with as little noise as possible. The street
had so recently roared to the noise of battle. All was now
hushed in silence. The presence of a single dead body
reminded them of the past. A dark heap of the slain also
reminded them. Clearly, the battle had been worse there.
They passed along the lanes and alleys opening into the
great street. They easily fancied the shadows of their
enemy lying in wait. The enemy looked ready to spring
upon them. But it was only fancy. The city slept without
being bothered by the rough rumbling of the cannons
and baggage trains. Even the constant sound of the tramp
of horses did not bother the city. At length, there was a
bright space beyond the dark line of the buildings. This
informed the army look-out of their coming out onto the
open highway. They might well have rejoiced. They had
thus escaped the dangers of an attack in the city itself. A
brief time would place them in greater safety on the
opposite shore.
Fig. 10. Original and revised samples of the passages used in the Duffy and Kabance study of 1981. Lack of
attention to coherence and other important variables can cancel out the effects of rewriting the text using the
readability-formula variables.
The Charrow and Charrow study Critics of the formulas (e.g., Bruce et al.
1981, Redish and Selzer; Redish 2000) also refer to the elaborate study of oral
jury instructions by attorney Robert Charrow and linguist Veda Charrow (1979).
They claimed that simplifying text did not make verbal instructions more
The authors did not use the readability variables in re-writing jury instructions
but simplified the instructions using a list of common legal “linguistic
constructions.” These were: nominalizations, unusual prepositional phrases,
misplaced phrases, whiz deletions (use of participles instead of verbs), deletions
of “that” or “which” beginning dependent clauses, technical legal terms,
imperative terms, negatives, passive voice, word lists, organization, and
dependent clauses.
The first experiment used 35 persons called for jury duty in Maryland using 14
jury instructions taken from California’s standard civil jury instructions. The
purpose of the study was mainly to see if it was the complexity of the legal issues
that made the instructions difficult or the difficulty of the language used. A group
of attorneys were asked to rate the instructions according to their perceived
The experimenters tested each person individually by playing each instruction
twice on a tape recorder. After hearing each instruction, the subject then verbally
paraphrased the instruction, which was also recorded. The results showed,
contrary to the attorneys’ expectations, it was not the complexity of the ideas that
caused problems in comprehension, but the difficulty of the language.
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Copyright © 2004 William H. DuBay
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The second experiment used 48 persons chosen for jury duty in Maryland. For
this experiment, they re-wrote the instructions, paying close attention to the legal
constructions noted above. They divided the group into two. Using 28 original
and modified instructions, they gave seven original instructions and seven
modified instructions to each group. They used the same protocols in playing the
instructions twice and asking the subjects to paraphrase them.
There was a significant improvement of the mean scores in comprehension in
nine of the fourteen instructions. They concluded that the subjects understood
the gist of the original only 45% of the time and the simpler ones 59% of the
This is not good enough, according to Professor Robert Benson (1984-85) of
Loyola Law School in Los Angeles. He wrote, “…none of us would care to be
tried by jurors who understood only 59% of their instructions.”
Benson went on to say that the Charrows own data was leading them to a
conclusion that they were unable to draw: that juries are never likely to
understand oral instructions adequately. Elwork, Sales, and Alfini (1982) reach
the same conclusion and recommend giving all jurors written as well as oral
To prove his point, Benson included three of the Charrows’ re-written
instructions in his own study on legal language using 90 law students and 100
non-lawyers. Using cloze tests, he found that, while the Charrows had reported
59% comprehension, the readers understood the written instructions almost fully
(p. 546).
As to the claim that paraphrasing is better than other testing techniques, Benson
claims that it has its own limitations, depending as it does on the subjects’ ability
to orally articulate what they understand. The Charrows had avoided asking the
subjects to paraphrase in writing because “subject’s writing skills would
confound the results.” Unfortunately, they ignored similar possible differences in
their listening and their oral skills (Benson, p. 537).
The Charrows state that sentence length does not cause reading difficulty.
“Although readability formulas are easy to use,” they write, “and certainly do
indicate the presence of lengthy sentences, they cannot be considered measures
of comprehensibility. Linguistic research has shown that sentences of the same
length may vary greatly in actual comprehensibility” (p. 1319).
Benson answered by writing that extremely long sentences such as those found
in legal language are known to cause difficulty, probably because of memory
constraints. He also found that the Charrows’ revised instructions had actually
shortened sentences by 35 percent. The shorter sentences “may well have played
a role in improved comprehension” (pp. 552-553).
A number of studies show that, in the average, as a sentence increases in length it
increases in difficulty (e.g., Coleman, 1962, Bormuth 1966). Average sentence
length has long been one of the clearest predictors of text difficulty.
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Copyright © 2004 William H. DuBay
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The New Readability Formulas
Critics of the formulas and formula developers questioned the reliability of the
criterion passages, criterion scores, and the reading tests on which the formulas
had been developed and validated. The arrival of cloze testing stimulated the
development of new criterion passages, new formulas, manual aids,
computerized versions, and the continued testing of text variables.
The Coleman formulas Edmund B. Coleman (1965), in a research project
sponsored by the National Science Foundation, published four readability
formulas for general use. The formulas are notable for their predicting mean
close scores (percentage of correct cloze completions).
Coleman was also the first to use cloze procedures as a criterion rather than the
conventional multiple-choice reading tests or rankings by judges.
The four formulas use different variables as shown here:
C% = 1.29w – 38.45
C% = 1.16w + 1.48s – 37.95
C% = 1.07s + 1.18s + .76p – 34.02
C% = 1.04w + 1.06s + .56p – .36prep – 26.01
Where: C% = percentage of correct cloze completions;
w = number of one-syllable words per 100 words
s = number of sentences per 100 words
p = number of pronouns per 100 words
prep = number of prepositions per 100 words
Coleman found multiple correlations of .86, .89, .90, and .91, respectively, for
his formulas with cloze criterion scores. The use of cloze scores as criterion
consistently provides higher validation coefficients than does use of the multiple-
choice scores. This may be a partial reason for the high correlations shown here.
The Bormuth studies Recognizing the problems of having more reliable
criterion passages, John Bormuth conducted several extensive studies, which
gave a new empirical foundation for the formulas. His first study (1966)
provided evidence of just how much changes in a number of readability variables
beside just vocabulary and sentence length can affect comprehension. Cloze
testing made it possible to measure the effects of those variables not just on the
difficulty of whole passages but also on individual words, phrases, and clauses.
His subjects included the entire enrollment of students (675) in grades 4 through
8 of Wasco Union Elementary School district in California. Their reading levels
went from the 2nd through the 12th grade. He used 20 passages of 275 to 300
words each, rated on the Dale-Chall formula from the 4th to the 8th-grade levels
of difficulty. He used five cloze tests for each passage, with the fifth-word
deletions starting at different words.
Reading researchers recognized that beginning readers relate differently to word
variables than do better readers. For this reason, special formulas have been
developed for the earliest primary grades such as the Spache formula (1953) and
the Harris-Jacobson primary readability formula (1973).
Bormuth’s study confirmed the curvilinearity of the formula variables. That
means their correlation with text difficulty changes in the upper grades,
producing a curve when plotted on a chart. Dale and Chall (1948) included an
adjustment for this feature in their formula-correction chart. This adjustment was
also included in the SMOG formula (McLaughlin 1968), the Fry Graph (Fry
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 44
1969), the FORCAST formula (Caylor et al. 1973), Degrees of Reading Progress
(Koslin et al. 1987), and the ATOS formula (Paul 2003).
Some critics of the formulas (Rothkopf 1972, Thorndike 1973-74, Selzer 1981,
Redish and Selzer 1985) claim that decoding words and sentences is not a
problem for adults. Bormuth’s study, however, shows that the correlation
between the formula variables and comprehension do not change as a function of
reading ability (p. 105). Empirical studies have confirmed that, in adult readers,
difficulty in reading is linked to word recognition (Stanovich 1984) and
decoding of sentences (Massad 1977). We cannot assume that adults are better
learners than children of the same reading level. In fact, they are often worse
(Russell 1973, Sticht 1982).
Bormuth’s next project (1969) was a study of the readability variables and their
relationship to comprehension. His subjects included 2,600 fourth-to-twelfth-
grade pupils in a Minneapolis school district.
The method consisted first in rating the reading ability of all the students with
the California 1963 Reading Achievement test. It used 330 different passages of
about 100 words each to confirm the reliability of 164 different variables, many
of them never examined before such as the parts of speech, active and passive
voice, verb complements, and compound nouns.
The five cloze tests used for each passage (resulting in 1,650 tests) gave him
about 276 responses for each deleted word, resulting in over 2 million responses
to analyze.
With this data, Bormuth was able to develop 24 new readability formulas, some
of which used 14 to 20 variables. These new variables, he found, added little to
the validity of the two classic-formula variables and were eventually dropped.
The study divided the students of each reading level into two groups, one that
was given a multiple-choice test and the other a cloze test of the same material.
Since Thorndike’s (1916) recommendation, educators and textbook publishers
had used 75% correct scores on a multiple-choice test as the criterion for
optimum difficulty for assisted classroom learning, and 90% for independent
reading. These criterion scores, however, were based on convention and use, not
on scientific study.
This Bormuth study validated the equivalencies of 35%, 45%, and 55% correct
cloze criterion scores with 50%, 75%, and 90% correct multiple-choice scores. It
also showed that the cloze score of 35% correct answers indicates the level of
difficulty required for maximum information gain.
Finally, this study produced three different formulas, one is for basic use, one for
machine use, and one for manual use. All three formulas predict the difficulty of
texts for all grade levels using a 35%, 45%, 55%, or a mean-cloze criterion.
The Bormuth Mean Cloze formula This formula uses three variables: number
of words on the original Dale-Chall list of 3,000, average sentence length in
words, and average word length in letters. This formula was later adapted and
used in the Degrees of Reading Power used by the College Entrance
Examination Board in 1981 (see below).
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Copyright © 2004 William H. DuBay
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The original Bormuth Mean Cloze formula is:
R = .886593 – .083640 (LET/W) + .161911 (DLL/W)3
– 0.021401 (W/SEN) + .000577 (W/SEN)2
– .000005 (W/SEN)3
DRP = (1 – R ) x 100
Where: R = mean cloze score
LET = letters in passage X
W = words in passage X
DLL = Number of words in the original Dale-Chall list in
passage X
SEN = Sentences in passage X
DRP = Degrees of Reading Power, on a 0-100 scale
with 30 (very easy) to 100 (very hard)
The findings of Bormuth about the reliability of the classic variables were
confirmed by MacGinitie and Tretiak (1971) who said that the newer syntactic
variables proposed by the cognitive theorists correlated so highly with sentence
length that they added little accuracy to the measurement. They concluded that
average sentence length is the best predictor of syntactic difficulty.
The Bormuth studies provided formula developers with a host of new criterion
passages. Critics of the formulas claimed that the criterion passages used by
formula developers were arbitrary or out-of-date (Bruce et al. 1981, Duffy,
1985). As new criterion passages became available, developers used them to
create new formulas and to correct and reformulate the older ones (Bormuth
1966, 1969, Klare 1985). The new Dale-Chall formula (1995) was validated
against a variety of criterion passages, including 32 developed by Bormuth
(1971), 36 by Miller and Coleman (1967), 12 by Caylor et al. (1973) and 80 by
MacGinitie and Tretiak (1971). Other formulas were validated against normed
passages from military technical manuals (Caylor et al. 1973, Kincaid et al.
The Fry Readability Graph While Edward Fry (1963, 1968) was working as a
Fullbright scholar in Uganda trying to help teachers teach English as a second
language, he created one of the most popular readability tests that use a graph.
Fig. 11. Edward Fry. His
Readability Graph may be the
most popular readability aid.
Fry would go on to become the director of the
Reading Center of Rutgers University and an
authority on how people learn to read.
Fry’s original graph determines readability
through high school. It was validated with
comprehension scores of primary and secondary
school materials and by correlations with other
Fry (1969) later extended the graph to primary
levels. In 1977, he extended it through the college
years (Fig. 11). Although vocabulary continues to
increase during college years, reading ability
varies much, depending on both individuals and
the subjects taught. That means that a text with a
score of 16 will be more difficult than one with a
score of 14. It does not mean, however, that one
is appropriate for all seniors and the other for all
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1. Select samples of 100 words.
2. Find y (vertical), the average number of sentences per 100-word
passage (calculating to the nearest tenth).
3. Find x (horizontal), the average number of syllables per 100-word
4. The zone where the two coordinates meet shows the grade score.
Fig. 12. The Fry Readability Graph as amended in 1977 with the extension
into the primary and college grades. Scores that appear in the dark areas
are invalid.
The Listening Formulas People have been concerned about the clarity of
spoken language perhaps for a longer period than written language. Speech is
generally much simpler than text. Because a listener cannot re-read a spoken
sentence, it puts a greater demand on memory. For this reason, “writing like you
talk” and reading text aloud have long been methods for improving readability.
Studies of the correlations of listenability and readability have had mixed results
(Klare 1963).
Some formulas have been developed just for spoken text. Rogers (1962)
published a formula for predicting the difficult of spoken text. He used 480
samples of speech taken from the unrehearsed and typical conversations of
students in elementary, middle, and high school as his data for developing his
formula. The resulting formula is:
G = .669 I + .4981 LD – 2.0625
G = reading grade level
I = average idea unit length
LD = the average number of words in a hundred-word sampling that do not
appear on Dale’s long list (3,000 words).
Roger’s formula has a multiple correlation of .727 with the grade level of his
The Principles of Readability
Copyright © 2004 William H. DuBay
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Irving Fang (1966-1967) used newscasts to develop his Easy Listening Formula
(ELF), shown here:
ELF = number of syllables above one per word in a sentence.
An average sentence should have an ELF score below 12 for easy listenability.
Fang found a correlation of .96 between his formula and Flesch’s Reading Ease
formula on 36 television scripts and 36 newspaper samples.
Subsequent research into listenability indicates that after the 8th grade, listening
skills do not keep up with the improvement in reading skills. After the 12th-grade
level, the same text may be harder to understand when heard than when read
(Chall 1983b; Dale and Chall 1995; Sticht, Beck, et al. 1974).
The SMOG formula G. Harry McLaughlin (1969) published his SMOG
formula in the belief that the word length and sentence length should be
multiplied rather than added. By counting the number of words of more than two
syllables (polysyllable count) in 30 sentences, he provides this simple formula:
SMOG grading = 3 + square root of polysyllable count.
McLaughlin validated his formula against the McCall-Crabbs passages. He used
a 100 percent correct-score criterion. As a result, his formula generally predicts
scores at least two grades higher than the Dale-Chall formula.
The FORCAST formula The Human Resources Research Organization studied
the reading requirements of military occupational specialties in the U.S. Army
(Caylor, Sticht, Fox, and Ford. 1973). In order to resolve professional questions
about using a formula for technical material read by adults, the authors first
undertook the creating of a readability formula that would be:
1. Based on essential Army-job reading material.
2. Adjusted for the young adult-male Army-recruit population.
3. Simple and easy for standard clerical personnel to apply without special
training or equipment.
The researchers first selected seven high-density jobs and 12 passages that
recruits are required to understand to qualify for them. They graded the passages
with the modified Flesch formula, finding them to range from the 6th to the 13th
grade in difficulty. They also selected 15 text variables to study for a new
formula. They next tested the reading ability of 395 Army recruits, and then
divided them into two groups, one with a mean-grade reading level of 9.40 and
another 9.42.
They next tested the recruits with cloze tests made of the 12 passages. The 12
passages were then re-graded using the criterion of at least 50% of those subjects
of a certain grade level being obtaining a cloze score of at least 35%. Results
indicated that average subjects scored 35.1% on the text graded 9.1 and 33.5%
on the text graded 9.6.
They next intercorrelated the results of the reading tests with the results of the
graded cloze tests. Results showed usable correlations of .83 and .75 for the two
groups of readers. Among the 15 variables they examined, the number of one-
syllable words in the passage correlated highest (.86) and was selected for use in
their new formula. Because they found that adding a sentence factor did not
improve the reliability of the formula, they left it out.
The Principles of Readability
Copyright © 2004 William H. DuBay
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The resulting FORCAST formula is:
Grade level = 20 – ( N ÷ 10 )
Where N = number of single-syllable words in a 150-word sample.
The new formula correlated r = 9.2 with the Flesch Reading Ease formula, 9.4
with the original Dale-Chall formula with, and r = .87 with the graded text
passages with. It is accurate from the 5th to the 12th grade.
They cross-validated the formula with a second study using another sample of
365 Army recruits at Ford Ord using another sample of reading passages scaled
from grade 7 to grade 12.7 using the FORCAST formula. The results of this
experiment correlated r = .98 with the Flesch formula, .98 with Dale-Chall, and
.77 with the graded military passages. These figures were judged appropriate for
the purpose of the formula.
Using the FORCAST formula, they tested the critical job-reading materials for
readability. The results show the percentage of materials in each occupation
written at the 9.9 grade level: Medical specialist, 24.4%; Light Weapons
Infantryman, 18.3%; Military Policeman, 15.1%; General Vehicle Repairman,
13.4%; Amorer/Unit Supply Specialist, 10.8%; Ground Control Radar
Repairman, 4.2%, and Personnel Specialist, 2.2%.
The study showed that materials for the different occupations all had texts above
the 9th grade. This suggested the need for new quality-control measures for
making materials more useful for the majority of personnel.
Fig. 13. Thomas Sticht. After participating
in the military studies which resulted in the
FORCAST readability formula, he went on
to become a worldwide authority in adult
literacy. He is shown here with UNESCO's
Mahatma Gandhi Medal he received in
2003 for his contributions to that field. The
citation reads:
"The UNESCO Mahatma Gandhi Bronze
Medal has been awarded to Dr. Thomas
Sticht (USA) in deep appreciation of his 25
years of service as a member of UNESCO's
International Jury for Literacy Prizes and in
recognition of his devotion to the cause of
adult literacy, especially for his efforts to
'reach the unreached.'"
In a follow-up study, Lydia Hooke and colleagues (1979) validated of the use of
the FORCAST formula on technical regulations for the Air Force. They also
found that four of seven writers of the regulations underestimated the grade level
of their materials by more than one grade.
In the main portion of the Hooke study, they administered cloze and reading tests
to 900 AF personnel to determine the comprehension of each regulation by the
user audience. Where there was no literacy gap (difficulty too high for the
reader), they found that comprehension was adequate (at least 40% cloze score)
in all cases. Where a literacy gap did exist, comprehension scores were below
the criterion of 40% in three of four cases.
The FORCAST formula is very unusual in that it does not use a sentence-length
measurement. This makes it a favorite, however, for use with short statements
and the text in Web sites, applications, and forms. The Department of the Air
The Principles of Readability
Copyright © 2004 William H. DuBay
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Force (1977) authorized the use of this formula in an instruction for writing
understandable publications.
The following are two of the scaled passages taken from training materials and
used in the occupational specialty study for the development and validation of
the FORCAST formula. Also shown are: 1. The scaled Reading Grade Level
(RGL), the mean reading grade level of the subjects who scored 35% correct
scores on the cloze tests; and 2. The scores of the FORCAST, the Flesch, and the
original Dale-Chall readability grade levels.
Passage 21
If you do not have a compass, you can find direction by other methods.
The North Star. North of the equator, the North Star shows you true
north. To find the North Star—
Look for the Big Dipper. The two stars at the end of the bowl are called
the “pointers.” In a straight line out from the pointers is the North Star
(at about five times the distance between the pointers). The Big Dipper
rotates slowly around the North Star and does not always appear in the
same position.
You can also use the constellation Cassiopeia. This group of five bright
stars is shaped like a lopsided M (or W, when it is low in the sky). The
North Star is straight out from the center star about the same distance
as from the Big Dipper. Cassiopeia also rotates slowly around the North
Star and is always almost opposite the Big Dipper.
Scaled RGL = 6. FORCAST = 8.6. Flesch = 7. Dale-Chall =7-8.
Passage 15
Adequate protection from the elements and environmental conditions
must be provided by means of proper storage facilities, preservation,
packaging, packing or a combination of any or all of these measures.
To adequately protect most items from the damaging effects of water or
water-vapors, adequate preservation must be provided. This is often
true even though the item is to be stored in a warehouse provided with
mechanical means of controlling the temperature and humidity. Several
methods by which humidity is controlled are in use by the military
services. Use is also made of mechanically ventilating and
dehumidifying selected sections of existing warehouses. Appropriate
consideration will be given to the preparation and care of items stored
under specific types of storage such as controlled humidity,
refrigerated, and heated. The amount and levels of preservation,
packaging, and packing will be governed by the specific method of
storage plus the anticipated length of storage.
Scaled RGL = 11.4. FORCAST = 12.1. Flesch = 13-16. Dale-Chall =
The Army’s Automated Readability Index (ARI) For the U.S. Army, Smith
and Senter (1967) created the Automated Readability Index, which used an
electric typewriter modified with three micro switches attached to cumulative
counters for words and sentences.
The ARI formula produces reading grade levels (GL):
GL = 0.50 (words per sentence) + 4.71 (strokes per word) – 21.43.
Smith and Kincaid (1970) successfully validated the ARI on technical materials
in both manual and computer modes.
The Navy Readability Indexes (NRI) Kincaid, Fishburne, Rogers, and
Chissom (1975, Fishburne 1976) followed a trend by recalculating new versions
of older formulas and testing them for use on Navy materials. The first part of
the experiment aimed at the recalculation of readability formulas. The second
The Principles of Readability
Copyright © 2004 William H. DuBay
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part of the study aimed at validating the effectiveness of the recalculated
formulas on Navy materials as measured by:
Comprehension scores on Navy training manuals
Learning time, considered being an important measurement of
The first part of the study first determined the reading levels of 531 Navy
personnel using the comprehension section of the Gates-MacGinitie reading test.
At the same time, they tested their comprehension of 18 passages taken from
Navy training manuals. The results of those tests were used in calculating the
grade levels of the passages. They then used those passages to recalculate the
ARI, Flesch, and Fog Count formulas for Navy use, now called the Navy
Readability Indexes (NRIs). The recalculated grade-level (GL) formulas are:
ARI simplified:
GL = .4 (words per sentence) + 6 (strokes per word) – 27.4
Fog Count new:
GL = ((easy words + 3 (hard words)) (sentences) ) – 3
easy words = number of number of 1 and 2-syllable words per 100
hard words = number of words of more than 2 syllables per 100 words
sentences = number of sentences per 100 words
Flesch Reading Ease formula simplified and converted to grade level (now
known as the Flesch-Kincaid readability formula):
GL = (.39 x ASL) + (11.8 x ASW) – 15.59
GL = ( .4 ASL ) + ( 12 ASW ) – 15
ASL = average sentence length (the number of words divided by the
number of sentences).
ASW = average number of syllables per word (the total number
syllables in the sample divided by the number of words).
The second part of the study looked at the relationship between readability and
learning time. It monitored the progress of 200 Navy technical-training students
through four modules of their course for both comprehension and learning time.
The study was replicated with a secondary sample of 100 subjects performing on
four additional modules.
The results of the comprehension test showed the highest percentage of errors in
both the readers with the lowest reading grade levels and in the modules with the
highest grade-levels of readability.
In the same manner, the learning time systematically decreased with reading
ability and increased with the difficulty of the modules. The study confirms that
learning time as well as reading ability are significant performance measures for
predicting readability.
The new Flesch-Kincaid formula was able to predict significant differences
between modules less than one grade level apart using both comprehension
The Principles of Readability
Copyright © 2004 William H. DuBay
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scores and learning times. The U.S. Department of Defense (1978) authorized
this formula in new procedures for validating the readability of technical
manuals for the Armed Services. The Internal Revenue Service, and the Social
Services Administration also issued similar directives.
Both Kern (1979) and Duffy (1985) urge the military to abandon use of the
formulas. They note that writers in the military often find the task of simplifying
texts below the 10th grade “too difficult” and “not worth the trouble.”
Unfortunately, there are no practical alternatives to the skill hard work required
to create simple language. When large numbers of readers are involved, even
small increases in comprehension pay off.
The Hull formula for technical writing At the 1979 STC conference, Leon C.
Hull (1979) argued that technical writing, with its increased use of difficult
words, needs a special kind of formula. While acknowledging that the
FORCAST and Kincaid formulas were developed precisely for that reason, he
looked for a formula that does not use word length as a variable.
Basing his work on Bloomer (1959) and Bormuth (1969) as well as his own
experience as a technical writer, Hull claims that an increase in the number of
adjectives and adverbs before a noun lowers comprehension. His study indicates
that the modifier load is almost as predictive as a syllable count, more causal,
and more helpful for rewriting.
Hull devised four cloze tests of each of five criterion passages from the Kincaid
study. The first test was the original passage. Each of the other tests increased
one of three indicators of modifier load by at least 50%: density of modifiers,
ambiguity of modifiers, and density of prepositions. The subjects were 107
science, engineering, and management students enrolled in a senior course in
technical and professional communication at Rensselaer Polytechnic Institute.
The mean cloze scores on the five unaltered passages correlated (r = ) 0.882 with
the Kincaid reading-grade levels assigned to these passages. This result justified
both the subject sampling and the use of the test results to produce a new
formula. The test results confirm the negative effect (r = -0.664) of modifier
density on comprehension. They also indicated that sentence length is a valid
indicator for technical material, perhaps better than word difficulty (contrary to
previous research).
Hull developed first formula with five variables, which accounts for (r2 = ) 68%
of passage difficulty. Like others before him, he found that the difficulty of using
a larger number of variables reduces the reliability of the formula and makes it
impractical. He created a another formula, shown here, that uses only sentence
length and the density of modifiers (called prenomial modifiers) and accounts for
(r2 = ) 48% of passage difficulty. Though slightly less valid than the Kincaid
formula, it is as accurate as many other popular formulas:
Grade level = 0.49 (average sentence length)
+ 0.29 (prenomial modifiers per 100 words) – 2.71
In the conclusion of his paper, Hull advises technical writers that using shorter
sentences reduces their complexity and makes them easier to read. He also
recommends eliminating strings of nouns, adjectives, and adverbs as modifiers.
Instead, writers should use prepositional phrases and place adjectives in the
predicate position (after the verb) rather than in the distributive position (before
the noun).
Degrees of Reading Power (DRP) In 1981, the College Entrance Examination
Board dropped its use of grade-level reading scores and adopted the Degrees of
The Principles of Readability
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Reading Power (DRP) system developed by Touchstone Applied Science
Associates (Koslin et al. 1987, Zeno et al. 1995).
The DRP uses the Bormuth Mean Cloze formula to predict scores on a 0 (easy)
to 100 (difficult) scale, which can be used for scoring both text readability and
student reading skills. The popular children's book Charlotte’s Web has a DRP
value of 50. Likewise, students with DRP test scores of 50 (at the independent
level) are capable of reading Charlotte’s Web and easier texts independently.
The Board also uses this system to provide readability reports on instructional
materials used by school systems.
Computerized writing aids Beginning in the 1980s, the first computer
programs appeared that not only contained the formulas but also other writing
aids. The Writer’s Workbench, developed at Bell Laboratories became the most
popular of these (Macdonald, Frase, Gingrich, and Keenan 1982). It contains
several readability indexes, stylistic analysis, average lengths of words and
sentences, spelling, punctuation, faulty phrases, percentages of passive verbs, a
reference on English usage, and many other features.
Kincaid, Aagard, O’Hara, and Cottrell (1981) developed CRES, a computer
readability editing system for the U.S. Navy. It contains a readability formula
and flags uncommon words, long sentences, and offers the writer alternatives.
Today, popular word processors such as Microsoft Word and Corel WordPerfect
include a combination of spell checkers, grammar checkers, and readability
formulas to help in creating texts that are more readable. Note that the Flesch-
Kincaid Grade Level in Word’s Readability Statistics is defective in that it only
goes to the 12th grade.
Lexile Framework At the height of the controversy about the readability
formulas, the founders of MetaMetrics, Inc. (Stenner, Horabin, et al. 1988a)
published a new system for measuring readability, Lexile Framework, which
uses average sentence length and average word frequency found in the American
Heritage Intermediate Corpus (Carroll et al. 1971) to predict a score on a 0–
2000 scale. The AHI corpus includes five million words from 1,045 published
titles to which students in grades three through nine are commonly exposed.
The cognitive theorists had claimed that different kinds of reading tests actually
measure different kinds of comprehension. The studies of the Lexile theorists
(Stenner et al. 1988b, Stenner and Burdick 1997) indicate that comprehension is
a one-dimensional ability that subsumes different types of comprehension (e.g.,
literal or inferential) and other reader factors (e.g., prior knowledge and special
subject knowledge). You either understand a passage or you don’t.
The New Dale-Chall Readability Formula In Readability Revisited: The New
Dale-Call Readability Formula, Chall and Dale (1995) updated their list of
3,000 easy words and improved their original formula, then 47 years old. The
new formula was validated against a variety of criteria, including:
32 passages tested by Bormuth (1971) on 4th to 12th-grade students.
36 passages tested by Miller and Coleman (1967) on 479 college
80 passages tested by MacGinitie and Tretiak (1971) on college and
graduate students.
12 technical passages tested by Caylor et al. (1973) on 395 Air Force
The new formula was also cross-validated with:
The Gates-MacGinitie Reading Test
The Principles of Readability
Copyright © 2004 William H. DuBay
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The Diagnostic Assessments of Reading and Trial Teaching Strategies
The National Assessment of Reading Progress
The Spache Formula
The Fry Graph
Average judgments of teachers on the reading level of 50 passages of
Fig 14. Jeanne S.
Chall. She created
the Harvard Reading
Lab and directed it for
20 years.
The new formula correlates .92 with the Bormuth Mean Cloze
Scores, making it the most valid of the popular formulas.
At the time of writing this, the new Dale-Chall formula is not yet
available on the Internet. It was once available in a computer
program, “Readability Master,” but is hard to find. You can easily
apply the formula manually, however, using the instructions,
worksheet, word list, and tables provided in the book. The book
also has several chapters reviewing readability research, the uses
of the formulas, the importance of vocabulary, the readability
controversies, and a chapter on writing readable texts.
The following are two of the sample passages in the book, with the
difficult words not found on their new word list underlined (pp.
135-140). The right-hand column gives a few readability statistics,
the New Dale-Chall mean cloze score, and reading grade level.
Grades 5-6
Eskimos of Alaska’s Arctic north coast
have hunted whales for centuries.
Survival has depended on killing the 80-
foot-long bowhead whales that swim from
the Bering Sea to the ice-clogged
Beaufort Sea each Spring. The Eskimos’
entire way of life has been centered
around the hunt.
But now that way of life is being
threatened by America’s need for oil, say
many Eskimos who hunt the whales.
Huge amounts of oil may be beneath the
Beaufort Sea. And oil companies want to
begin drilling this spring.
However, many Eskimos say severe
storms and ice conditions make drilling
From My Weekly Reader, Edition 6
Readability Data
Number of Words in Sample 100
Number of Whole Sentences 6
Number of Unfamiliar Words 11
Cloze Score 42
Reading Level 5-6
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 54
Grades 9-10
The controversy over the laser-armed
satellite boils down to two related
questions: Will it be technically effective?
And should the United States make a
massive effort to deploy it?
To its backers, the laser seems the
perfect weapon. Traveling in a straight
line at 186,000 miles per second, a laser
beam is tens of thousands of times as
fast as any bullet or rocket. It could strike
its target with a power of many watts per
square inch. The resulting heat, combined
with a mechanical shock wave created by
recoil as surface layers were blasted
away, could quickly melt…
From Discover
Readability Data
Number of Words in Sample 100
Number of Whole Sentences 5
Number of Unfamiliar Words 23
Cloze Score 28
Reading Level 9-10
ATOS readability formula for books Researchers at School Renaissance
Institute (1999, 2000, Paul 2003) and Touchstone Applied Science Associates
produced the Advantage-TASA Open Standard (ATOS) Readability Formula for
Books. Their goal was to create an “open” formula that would be available to the
educational community free of charge, that would be easy to use, and that could
be used with any nationally normed reading tests.
The project was perhaps the most extensive study of readability ever conducted.
Formula developers used 650 norm-referenced reading tests, 474 million words
representing all the text of 28,000 K–12 books read by real students with many
published in the previous five years, an expanded vocabulary list, and the reader
records of more than 30,000 students who read and tested on 950,000 actual
The readability formula was part of a computerized system to help teachers
conduct a program of guided independent reading to maximize learning gains.
Noting the differences in difficulty between samples and entire books, the
developers claim this is the first readability formula based on whole books, not
just samples.
They found that the combination of three variables gives the best account of text
difficulty: words per sentence (r2 = .897), the average grade-level of words (r2 =
.891), and characters per word (r2 = .839). The formula produces grade-level
scores, as they are easier for teachers to understand and use.
The formula developers paid special attention to the Zone of Proximal
Development (ZPD) proposed by Vygotsky (1978), the level of optimal
difficulty that produces the most learning gain. They found that, for independent
reading below the 4th grade, maximum learning gain requires at least 85%
comprehension. Advanced readers need a 92% score on reading quizzes. Those
who exceed that percentage should be given material that is more challenging.
Other results of the studies indicate that:
Maximum learning gain requires careful matching of book readability
and reading skill.
The amount of time spent reading correlates highly with gains in
reading skill.
Book length can be a good indication of readability.
Feedback and teacher interaction are the most important factors in
accelerated reading growth.
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 55
Formula Applications
Many researchers outside the field of reading have recognized the value of the
formulas. Edward Fry (1986) points out that articles on the readability formulas
are among the most frequently cited articles of all types of educational research.
The applications give researchers an objective means of controlling the difficulty
of passages in their experiments.
The following is a sample of readability studies that used formulas: political
literature (Zingman 1977), corporate annual reports (Courtis 1987), customer
service manuals (Squires and Ross 1990) drivers’ manuals (Stahl and Henk
1995), dental health information (Alexander 2000), palliative-care information
(Payne et al. 2000), research consent forms (Hochhauser 2002; Mathew 2002;
Paasche-Orlow et al. 2003), informed consent forms (Williams et al. 2003)
online health information (Oermann and Wilson 2000), lead-poison brochures
(Endres et al. 2002) online privacy notices (Graber et al. 2002) medical journals
(Weeks and Wallace 2002), environmental health information (Harvey and
Fleming 2003) and mental-health information (King et al. 2003).
Court actions and legislation Fry (1989a) points out that the validity of the
formulas has been challenged in court and found suitable for legal purposes. The
courts increasingly rely on readability formulas to show the readability of texts in
protecting the rights of citizens to clear information. Court cases and legislation
involving government documents and correspondence, criminal rights, product
labeling, private contracts, insurance policies, ballot measures, warranties, and
warnings are some of the legal applications of the formulas.
In 1984, Joseph David of New York was upset by his inability to understand a
letter of denial he received in response to his appeal for Medicare benefits. Legal
Services went to court in behalf of David and other elderly recipients of
Medicare in New York. They pointed out that 48% of the population over 65
had less than a 9th-grade education. Edward Fry testified in court that the denial
letter was written at the 16th-grade level. As a result, the judge ordered the
Secretary Heckler of the U.S. Department of Health and Social Services to take
“prompt action” to improve the readability of Medicare communications (David
vs. Heckler 1984).
A number of federal laws require plain language such as the Truth in Lending
Act, the Civil Rights Act of 1964, and the Electronic Funds Transfer Act. In June
1998, President Clinton directed all federal agencies to issue all documents and
regulations in plain language.
Beginning in 1975, a number of states passed plain-language laws covering such
common documents as bank loans, insurance policies, rental agreements, and
property-purchase contracts. These laws often state that if a written
communication fails the readability requirement, the offended party may sue and
collect damages. Such failures have resulted in court judgments.
States such as California also require plain language in all agency documents,
including “any contract, form, license, announcement, regulation, manual,
memorandum, or any other written communication that is necessary to carry out
the agency's responsibilities under the law” (Section 6215 of the California
Government Code). California defines plain language as “written or displayed so
that the meaning of regulations will be easily understood by those persons
directly affected by them” (Section 11349 of the Administrative Code).
Textbook publishers After 80 years, textbook publishers consider the grade
level of textbooks as more important than cost, the choice of personnel, or the
physical features of books. All of them use word-frequency lists. Eighty-nine
percent of them use readability formulas in evaluating the grade-levels of texts,
The Principles of Readability
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along with other methods of testing. Widely read children’s publications such as
My Weekly Reader and magazines published by National Geographic for
children of different ages have used the formulas along with field-testing and
other methods (Chall and Conard 1991).
Using the Formulas
Formula discrepancies The discrepancy between the scores of different
formulas has long been perplexing. For example, the scores for the following
four paragraphs are:
Original Dale-Chall grade level: 11-12
Flesch grade level: 8.9
FORCAST grade level: 10.9
SMOG grade level: 11.7
Fog grade level: 12.3
Critics have often cited such discrepancies as indications of the lack of precision
of the formulas. Kern (1979) argued that the discrepancies among the Kincaid
and Caylor formulas deprive them of usefulness, and that the military should
discard them. What Kern ignores in his review are the correlations of the
formulas with comprehension tests. What is important is not how the formulas
agree or disagree on a particular text, but their degree of consistency in
predicting difficulty over a range of graded texts.
The most obvious causes of the discrepancies are the different variables used by
different formulas and the different criterion scores used in their development.
The formulas—like reading tests—simply do not have a common zero point
(Klare 1982). The criterion score is the required level of comprehension
indicating reading success as indicated by the percentage of correct answers on a
reading test. For example, a formula can predict the level of reading skill
required to answer correctly 75 % of the questions on a reading test based on a
criterion passage.
The FORCAST and Dale-Chall formula use a 50% criterion score as measured
by multiple-choice tests. The Flesch formula use a 75% score, Gunning Fog
formula, a 90% score, and the McLaughlin SMOG formula a 100% score. The
formulas developed with the higher criterion scores tend to predict higher scores,
while those the highest validity correlations (e.g., Dale-Chall and Flesch) tend to
predict lower scores.
The different methods used by different computer programs to count sentences,
words, and syllables can also cause discrepancies—even though they use the
same formula. Finally, the range of scores provided by different formulas remind
us that they are not perfect predictors. They provide probability statements or
rather estimates of difficulty.
The problem of optimal difficulty Different uses of a text require different
levels of difficulty. As we have seen, Bormuth (1969) indicated the 35% cloze
score was the point of optimum learning gain (see Table 7 above) for assisted
classroom reading.
Vygotsky (1978) supported Bormuth’s findings that optimal difficulty should be
slightly above their current level of development and not below. Using books
that are at the reader’s present level or below may increase fluency and rate, but
not in the way of comprehension.
For this reason, reading experts advise that materials intended for assisted
reading when an instructor is available should be somewhat harder than the
readers’ tested reading level. Materials for the general public, however, such as
medicine inserts, instructions for filing tax forms, instructions for using
The Principles of Readability
Copyright © 2004 William H. DuBay
Page 57
appliances, and health information should be as easy as possible to convey
(Chall and Dale 1995).
Paul (2003) found that independent reading requires at least an 85%
comprehension on multiple-choice reading quizzes for readers below the 4th
grade and 92% for advanced readers. He also recommends that advanced
students who score better than 92% correct on quizzes should be given material
that is more challenging.
The formulas and usability testing Redish (2000) and Schriver (1991, 2000),
promote the need for reading protocols and usability testing as an alternative to
the formulas. They feel that usability testing eliminates the need for readability
testing. They fail to state, however, how to match the reading ability of subjects
with that of the target audience.
Dumas and Redish (1999), in their work on usability testing, hardly mention
reading comprehension. They have us assume that, if test subjects correctly
perform a task, they have correctly understood the instructions. When problems
arise, however, it is difficult to locate the source of the difficulty.
In both usability testing and reading protocols, some subjects are more skilled
than others in articulating the problems they encounter. Do problems come from
the text or from some other source? If they are located in the text, do they come
from the design, style, organization, coherence, or content? We are often left
with guesswork and trial-and-error cycles of revision and testing.
As experience has taught us, this gets expensive. In developing a text, it makes
as little sense to neglect the readability of a document as it does to neglect its
punctuation, grammar, coherence, or organization. Readability is not a trivial
issue. If an audience cannot read a text, they cannot understand it. If they cannot
understand it, they cannot use it to complete a task.
Schriver (1997) and Hackos and Redish (1998) correctly emphasize the
importance of testing and of frequent consultation with members of the targeted
audience before, during, and after developing a document. Assessing both the
reading ability of the audience and the readability of the text will greatly
facilitate this process.
Today, the readability formulas are more popular than ever. There are
readability formulas for Spanish, French, German, Dutch, Swedish, Russian,
Hebrew, Hindi, Chinese, Vietnamese, and Korean (Rabin 1988).
The formulas have survived 80 years of intensive application, investigation, and
controversy, with both their credentials and limitations remaining intact. The
national surveys on adult literacy have re-defined our audience for us. Any
approach to effective communication that ignores these important lessons cannot
claim to be scientific. If we walk away from this research, others will one day
rediscover it and apply it to our work as technical communicators.
The variables used in the readability formulas show us the skeleton of a text. It is
up to us to flesh out that skeleton with tone, content, organization, coherence,
and design. Gretchen Hargis of IBM (2000) states that readability research has
made us very aware of what we “write at the level of words and sentences.” She
Technical writers have accepted the limited benefit that these
measurements offer in giving a rough sense of the difficulty of
The Principles of Readability
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We have also assimilated readability as an aspect of the quality of
information through its pervasiveness in areas such as task orientation,
completeness, clarity, style, and visual effectiveness. We have put into
practice, through user-centered design, ways to stay focused on the
needs of our audience and their problems in using the information or
assistance that we provide with computer products.
The research on literacy has made us aware of the limited reading abilities of
many in our audience. The research on readability has made us aware of the
many factors affecting their success in reading. The readability formulas, when
used properly, help us increase the chances of that success.
Copyright © 2004 William H. DuBay
Page 59
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