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The Impact of Augmentative and Alternative Communication Intervention on the Speech Production of Individuals With Developmental Disabilities: A Research Review

American Speech-Language-Hearing Association
Journal of Speech, Language, and Hearing Research
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Abstract

Purpose This article presents the results of a meta-analysis to determine the effect of augmentative and alternative communication (AAC) on the speech production of individuals with developmental disabilities. Method A comprehensive search of the literature published between 1975 and 2003, which included data on speech production before, during, and after AAC intervention, was conducted using a combination of electronic and hand searches. Results The review identified 23 studies, involving 67 individuals. Seventeen of these studies did not establish experimental control, thereby limiting the certainty of evidence about speech outcomes. The remaining 6 studies, involving 27 cases, had sufficient methodological rigor for the “best evidence analysis” (cf. R. E. Slavin, 1986). Most of the participants (aged 2–60 years) had mental retardation or autism; the AAC interventions involved instruction in manual signs or nonelectronic aided systems. None of the 27 cases demonstrated decreases in speech production as a result of AAC intervention, 11% showed no change, and the majority (89%) demonstrated gains in speech. For the most part, the gains observed were modest, but these data may underestimate the effect of AAC intervention on speech production because there were ceiling effects. Conclusions Future research is needed to better delineate the relationship between AAC intervention and speech production across a wider range of participants and AAC interventions.
Diane C. Millar
The Pennsylvania State University,
University Park, and
University of North Texas, Denton
Janice C. Light
The Pennsylvania State University,
University Park
Ralf W. Schlosser
Northeastern University,
Boston, Massachusetts
The Impact of Augmentative
and Alternative Communication
Intervention on the Speech Production
of Individuals With Developmental
Disabilities: A Research Review
Purpose: This article presents the results of a meta-analysis to determine the effect of
augmentative and alternative communication (AAC) on the speech production of
individuals with developmental disabilities.
Method: A comprehensivesearch of the literature published between1975 and 2003,
which included data on speech production before, during, and after AAC intervention,
was conducted using a combination of electronic and hand searches.
Results: The review identified 23 studies, involving 67 individuals. Seventeen of these
studies did not establish experimental control, thereby limiting the certainty of evidence
about speech outcomes. The remaining 6 studies, involving 27 cases, had sufficient
methodological rigor for the ‘best evidence analysis’ (cf. R. E. Slavin, 1986). Most of
the participants (aged 2–60 years) had mental retardation or autism; the AAC
interventions involved instruction in manual signs or nonelectronic aided systems. None
of the 27 cases demonstrated decreases in speech production as a result of AAC
intervention, 11% showed no change, and the majority (89%) demonstrated gains in
speech. For the most part, the gains observed were modest, but these data may
underestimate the effect of AAC intervention on speech production because there were
ceiling effects.
Conclusions: Future research is needed to better delineate the relationship between
AAC intervention and speech production across a wider range of participants andAAC
interventions.
KEY WORDS: augmentative and alternative communication, speech production,
developmental disabilities, effectiveness, meta-analysis, systematic review
Augmentative and alternative communication (AAC) interven-
tions can benefit individuals with developmental disabilities who
have significant speech and language impairments by enhancing
their communicative competence (e.g., Light, Binger, Agate, & Ramsay,
1999) and facilitating the development of language skills (e.g., Romski
& Sevcik, 1996). Despite these recognized benefits, some parents and
professionals are hesitant to initiate AAC interventions because of
concerns that AAC will inhibit speech production (e.g., Beukelman, 1987;
Silverman, 1995). They worry that AAC may become a ‘‘crutch’’ for
individuals with developmental disabilities, negatively impacting the
emergence of speech (Dowden & Marriner, 1995), and argue that
Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006 AAmerican Speech-Language-Hearing Association248
1092-4388/06/4902-0248
individuals with developmental disabilities may prefer
to use AAC and may not be motivated to learn to use
speech to communicate, because they perceive that AAC
is an easier way to communicate compared with speech
(Glennen & DeCoste, 1997).
Others (Romski & Sevcik, 1996) have proposed a
counterargument, positing that AAC will actually facil-
itate the production of speech for individuals with
developmental disabilities who have significant speech
impairments. Those who support this counterargument
outline a number of reasons why AAC would benefit
speech production. First, AAC intervention may reduce
the pressure on the individual for speech production,
thereby reducing stress and indirectly facilitating speech
(Lloyd & Kangas, 1994). Second, AAC intervention may
allow individuals with significant speech impairments to
bypass the motor and cognitive demands of speech
production and focus on the goal of communication
instead; after they establish basic communication and
language skills, they may then be better able to reallocate
resources to improve their speech productions (Romski &
Sevcik, 1996). Proponents of this argument emphasize
that speech is a much more efficient means of commu-
nication compared with AAC and believe that children
will inevitably choose the easier, more efficient, and more
accepted mode of communication (i.e., speech), provided
it is a viable mode within their repertoire.
According to Mirenda (2003), the behavioral theory
of automatic reinforcement may provide another poten-
tial explanation of the facilitative effects of AAC inter-
vention on speech development. According to this theory,
if AAC (e.g., the manual sign or graphic symbol for
cookie) is presented along with the spoken word (as is
typically the case in AAC intervention), and these are
followed by a reinforcer (e.g., a chocolate chip cookie),
both the AAC mode and speech production should in-
crease in frequency.
Others have argued that AAC facilitates speech
production because AAC provides a more immediate
and consistent model for individuals with developmental
disabilities; this is particularly the case when AAC
involves speech output (synthesized or digitized) as a
model for speech production (Blischak, 2003; Romski &
Sevcik, 1996; M. Smith & Grove, 2003).
Professionals and parents urgently need empirical
evidence about the impact of AAC on speech production
to make informed decisions about AAC interventions. In
1995, Silverman published a review of the literature ad-
dressing the impact of AAC on speech production. This
review is frequently quoted as evidence that AAC fa-
cilitates speech production. Silverman reported that he
reviewed over 100 published and unpublished reports to
determine the impact of AAC on speech production. He
concluded that, ‘‘Theuse of augmentativeand alternative
communication seems to facilitate speech (i.e., increase
verbal output) in some children and adults’’ (p. 34).
Furthermore, he concluded that at least 40% of the in-
dividuals in the studies he reviewed demonstrated an
increase in speech as a result of AAC interventions. How-
ever, examination of the primary sources included in
Silverman’s review reveals several critical limitations
that make it difficult to draw reliable conclusions about
the impact of AAC on speech production. First, there is no
description of the selection criteria or search procedures
used to identify studies for the review. Second, references
are provided for less than a third of the 100 reports re-
viewed (Silverman, 1995). In addition, of the references
provided, many of the sources are unpublished; others
that are published do not report reliable data on the
participants’ speech productions before and after AAC
intervention. Furthermore, analysis of the data from
some of the published studies suggests some inaccuracies
in the conclusions reported (e.g., conclusions that there
was an increase in speech attempts forsome participants
were not valid). Last, most of the studies cited in
Silverman’s review were published prior to 1982. There
have been many advances in AAC interventions since
that time.
There is an urgent need for a systematic review of
the current research on AAC intervention and natural
speech production to guide evidence-based decision
making in initiating AAC (Schlosser & Raghavendra,
2004). Our goal in this article is to report the results of a
systematic research review designed to determine the
relationship between AAC intervention and speech de-
velopment in individuals with developmental disabilities.
Method
Inclusion Criteria
There were a number of criteria specified for the
inclusion of studies in the review: (a) The studies were
published between 1975 and 2003, (b) they involved in-
dividuals with developmental disabilities who had signif-
icant speech impairments (i.e., speech was not adequate
to meet daily communication needs), (c) they included
implementation of AAC (i.e., the compensation for
spoken or written communication through unaided or
aided AAC systems; Lloyd, Fuller, & Arvidson, 1997) and
they included documentation of progress in the acqui-
sition of the use of AAC, and (d) they included data on
speech production for the participants before, during,
and/or after AAC intervention (e.g., number of words
spoken). Speech production was defined as the oral
expression of language (Hulit & Howard, 2002) and
included oral production of intelligible words or word
approximations understood in context. Studies of indi-
viduals whose primary impairment was a hearing
Millar et al.: Natural Speech and AAC 249
impairment were excluded from the review because
these individuals are typically not considered within the
population of individuals who require AAC (American
Speech-Language-Hearing Association, 1991). Studies of
individuals who had acquired disabilities were excluded;
AAC interventions might be expected to impact the
speech recovery of individuals with acquired disabilities
in different ways than these interventions would impact
the speech development of individuals with developmen-
tal disabilities who had no history of age-appropriate
speech skills. Studies that included AAC interventions
but did not document the participants’ acquisition of AAC
were also excluded (e.g., Yoder & Layton, 1988).
Search Procedures
A multifaceted search strategy was used to locate all
studies that met the selection criteria and to avoid a
biased yield, as would occur if only one source were con-
sulted (White, 1994). First, electronic searches of various
databases (i.e., PsycINFO, ERIC, Medline) were con-
ducted using keywords (i.e., ‘‘nonspeaking,’’ ‘‘nonvocal,’’
‘‘augmentative communication,’’ ‘‘speech production,’’
‘‘speech development’’). Next, hand searches or electronic
searches of the tables of contents of 46 journals were
performed (see Appendix A). A comprehensive list of
journals was developed that had a history of including
articles onAAC. The list of journals included in Schlosser
and Lee’s (2000) meta-analysis was also used as a guide
to generate the list of journals for the current review.
Last, ancestral searches of references cited in studies
that met the selection criteria were conducted. These
search procedures were designed to maximize theyield of
relevant studies; however, as with any search, it is
possible that a relevant study was missed.
Reliability of the search procedures. Five of the 46
journals (11%) were randomly selected and searched
either by hand or electronically by a graduate student to
determine the reliability of the search procedures. A total
of 3,089 articles were reviewed using the inclusion
criteria described above. The interjudge reliability was
nearly 100% for the search procedures (i.e., 3,088 agree-
ments out of 3,089 total articles). The single discrepancy
was discussed and resolved.
Coding Procedures
Each study that met the selection criteria was
reviewed and coded. The coding categories were based,
in part, on the categories used by Schlosser and Lee
(2000) in their meta-analysis of treatment effectiveness
of AAC interventions. Additional categories specific to
the objectives of the present review were added on the
basis of a review of the related literature and input from
experts in AAC. (See Appendix B for the final coding
categories and their operational definitions.)
Each study was coded with respect to (a) the goals of
the study, (b) the design, (c) the participants (i.e., gender,
disability, chronological age), (d) the independent vari-
able (i.e., type of AAC system, intervention procedures,
number of sessions), (e) the percentage of nonoverlap-
ping data (PND) of the speech production data in the case
of single-participant experimental designs (Scruggs,
Mastropieri, & Casto, 1987) or the effect size in the case
of group designs (Kazdin, 2003), and (f) the speech out-
comes (i.e., the changein speech production calculated as
the amount of increase or decrease in speech productions
during or after the AAC intervention compared with
baseline preintervention). The PND and speech out-
comes were coded for each participant in each single-
participant experimental design separately according to
the procedures established by Schlosser and Lee (2000).
When participants were involved in more than one
application of the same AAC intervention (e.g., as in a
multiple-baseline-across-behaviors design where the
same AAC intervention was applied across several sets
of stimuli or in an ABAB withdrawal design), the re-
searchers reported the mean PND of the AAC interven-
tion across the applications according to the procedures
suggested by Scruggs et al. (1987). If participants were
involved in more than one treatment (as in an alternat-
ing treatments design), PND and speech outcomes for the
individual treatments were coded as separate cases for
each participant. This allowed for analysis of changes in
speech productions with different interventions. In some
of the alternating treatment designs (e.g., Conaghan,
Singh, Moe, Landrum, & Ellis, 1992; Linton & Singh,
1984; Sisson & Barrett, 1984), after the more effective
treatment was established, it was then applied to the less
effective treatment conditioninanewphase(C)ofthe
study. The application of the more effective AAC treatment
to the less effective condition was omitted from the anal-
ysis because the effects of AAC on speech production could
not be reliably determined because of the confounding ef-
fects of the intervening, less effective treatment condition.
The increases or decreases in speech were quantified
using the unit of measure adopted in each study (e.g., five
spoken words) according to the following formula: maxi-
mum point during or following AAC intervention minus
maximum point at baseline. The speech gains (or de-
creases) were reported separately for each application of
an AAC intervention. Unfortunately, the studies did not
use a common metric for measuring speech gains. Some
studies measured intelligible words produced; others
measured oral production of short, multiword phrases;
and some used time-interval data and measured speech
production as the percentage of opportunities with speech
productions (e.g., Charlop-Christy, Carpenter, Loc,
250 Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006
LeBlanc, & Kellet, 2002). As a result, it was not possible to
aggregate speech gains precisely across studies.
Last, the methodological rigor of each study was eval-
uated on the basis of the level of experimental control, the
reliability of the dependent variable, and the treatment
integrity. Specifically, the methodological quality of each
study was coded according to the certainty of evidence
(cf. Simeonsson & Bailey, 1991; N. L. Smith, 1981) pro-
vided with respect to the effect of AAC intervention on
speech production: (a) conclusive evidence (i.e., the de-
sign provided experimental control, the dependent vari-
able was reliable, and treatment integrity was solid,
allowing the conclusion that the speech outcomes for the
participant were undoubtedly the result of the AAC in-
tervention); (b) preponderant evidence (i.e., the study had
minor flaws with respect to the design, reliability of the
dependent variable, or treatment integrity, resulting in
the conclusion that speech outcomes were more likely to
have occurred as a result of the AAC intervention than
not, but the evidence was not conclusive); (c) suggestive
evidence (i.e., the study had several minor flaws, leading
to the conclusion that it was plausible, but not certain,
that speech outcomes were the result of the AAC inter-
vention); and (d) inconclusive evidence (i.e., there were
significant flaws in the design that precluded any con-
clusions regarding the impact of the AAC intervention on
speech production). See Appendix B for operational defi-
nitions of the coding categories. This classification sys-
tem provided a systematic method for documenting the
level of certainty of evidence when evaluating the impact
of interventions (Simeonsson & Bailey, 1991) and al-
lowed us to determine which studies provided the best
evidence.
Conclusive evidence may not always be available,
particularly in the formative stages of evaluating specific
phenomena. N. L. Smith (1981) argued that evidence at
lower levels of certainty should be considered in these
cases and concluded that such evidence is ‘‘certainly better
than no informed conclusions at all’’ (p. 278), provided the
intervention does not pose significant risk for the partic-
ipants. In situations where conclusive evidence is not
available, the best evidence availableshouldbeconsidered.
Best Evidence Analysis
In this research review, we applied a best evidence
analysis (cf. Slavin, 1986) on the basis of the certainty
of evidence provided by the studies. A best evidence ap-
proach combines the analysis of quantitative outcome
measures with an evaluation of the quality of individual
studies and the certainty of the evidence that the studies
provide (Slavin, 1986). Greater weight is given to the best
evidence available; that is, greater weight is given to those
studies that use more rigorous designs and, thereby,
provide greater certainty of evidence. Studies that are
flawed methodologically are identified as such and
given less weight in drawing summary conclusions
(McNaughton, 1994). A best evidence analysis was es-
pecially relevant in the present research review because
of the variability in methodological rigor across the studies
reviewed. Many of the studies failed to establish exper-
imental control with respect to the relationship of interest
in this review—the effect of AAC intervention as an inde-
pendent variable on speech production as a dependent
variable. In most of these studies, the relationship be-
tween AAC intervention and speech production was not
the primary research question in the studies; rather, data
on speech outcomes were collected as secondary or col-
lateral measures. As a result, the certainty of evidence in
these studies was inconclusive and the effects of AAC
intervention on speech productions could not be reliably
determined. Greater weight was therefore accorded to
those studies that established experimental control and
allowed more reliable investigation of the relationship be-
tween AAC intervention and speech production.
Reliability of the coding. To ensure reliability of the
coding, 20% of the studies were reviewed and coded in-
dependently by a second trained coder. Mean interrater
reliability (number of agreements divided by number
of agreements plus number of disagreements plus omis-
sions) across the studies was 98% (range = 90%–100%).
The mean interrater reliability for each variable was as
follows: (a) goals, 100%; (b) design, 100%; (c) participants,
100%; (d) AAC intervention, 97%; (e) effect of AAC inter-
vention, 100%; and (f) changes in speech production,
90%. Any discrepancies were resolved through discus-
sion before the coding was finalized.
Results
The search resulted in the identification of 23 stud-
ies that met the inclusion criteria (see Table 1). Of the
23 studies, 8 were descriptive case studies; 6 were
single-participant, alternating treatment designs;
6 were single-participant, multiple baseline designs;
1 was a single-participant, alternating treatment design
within a multiple baseline; 1 was a single-participant
withdrawal design; and 1 was a group pretest–posttest
design. The studies involved a total of 67 participants:
40% had mental retardation, 31% had autism, and the
rest had other disabilities (e.g., Klinefelter’s syndrome,
cerebral palsy). The goal of 70% of the studies was to
teach expressive vocabulary, either in the form of single
words (44%) or in short phrases (26%); the goal of the
remaining studies (30%) was to teach the expression of
various communicative functions (e.g., requests, com-
ments). The majority (61%) of the studies investigated
unaided AAC interventions (specifically, manual signs);
31% investigated nonelectronic-aided AAC systems;
Millar et al.: Natural Speech and AAC 251
Table 1. Studies, published between 1975 and 2003, involving AAC interventions with individuals with developmental disabilities that
documented speech production before and during/after intervention.
Study/design Goal No. of participants and age
a,b
AAC intervention
Authors’ conclusions re.
speech (no. of participants)
*Barrett & Sisson (1987) Teach two or more 2 participants with MR Unaided Increase (2)
Alternating treatments
within a multiple
baseline
word combinations 5 & 13 years old
Benaroya, Wesley, Ogilvie,
Klein, & Meaney (1977)
Teach two or more
word combinations
6 participants with autism
5–12 years old
Unaided Increase (3)
No change (3)
Case study
Bondy & Frost (1994)
Case study
Teach function 1 participant with autism
3 years old
Aided with no
speech output
Increase (1)
Blischak (1999)
Pretest–posttest
group design
Teach function 1 participant with Down syndrome,
2 with multiple disabilities,
1 with cerebral palsy, and
5 with dysarthria and/or
apraxia (no definitive diagnoses)
4–7 years old
Aided with speech
output and aided
without speech output
Aided (with no speech output):
increase (2), decrease (2)
Aided (with speech output):
increase (4), decrease (1)
Bonta & Watters (1983) Teach single words 1 participant with autism Unaided Increase (1)
Case study 11 years old
Casey (1978) Teach single words 4 participants with autism Unaided Increase (4)
Multiple baseline 6–7 years old
*Charlop-Christy
et al. (2002)
Teach function 3 participants with autism
3–12 years old
Aided with no
speech output
Increase (3)
Multiple baseline
Clarke, Remington, &
Light (1986)
Teach single words 1 participant with MR
6 years old
Unaided Increase (1)
Alternating treatments
Clarke, Remington, &
Light (1988)
Teach single words 2 participants with MR
6 & 7 years old
Unaided Increase (2)
Alternating treatments
*Conaghan et al. (1992)
Alternating treatments
Teach two or more
word combinations
4 participants with MR
and hearing impairments
Unaided Increase (3)
No change (1)
18–60 years old
Cregan (1993)
Case study
Teach two or more
word combinations
1 participant with MR
14 years old
Unaided and aided
with no speech output
Increase (1)
DiCarlo, Stricklin, &
Banajee (2001)
Multiple baseline
Teach single words 6 participants with Down
syndrome, autism, or
cerebral palsy
1–3 years old
Unaided Increase
c
Fulwiler & Fouts (1976) Teach single words 1 participant with autism Unaided Increase (1)
Case study 5 years old
(Continued on the following page)
252 Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006
1 study (4%) investigated a combination of aided AAC
systems with speech output and aided AAC systems
without speech output; and 1 study (4%) investigated
multimodal interventions combining unaided and aided
AACs without speech output. The authors of the 23
studies reported the following conclusions regarding the
effects of AAC intervention on speech production: (a)
Speech increased for 55 of the 67 participants (82%),
Garrison-Harrel, Kamps,
& Kravits (1997)
Teach function 3 participants with autism
6–7 years old
Aided with no
speech output
Increase (3)
Multiple baseline
Gibbs & Carswell (1991) Teach single words 1 participant with MR Unaided Increase (1)
Case study 1 year old
Johnston, Nelson, Evans,
& Palazolo (2003)
Teach function 3 participants with MR
4–5 years old
Aided with no
speech output
Increase (3)
Multiple baseline
*Kouri (1988)
Withdrawal
Teach single words 1 participant with autism, 2
with Klinefelter’s syndrome,
1 with Down syndrome, and
1 with an unknown diagnosis
Unaided Increase (3)
Decrease (2)
2–4 years old
Kravits, Kamps,
Kemmerer, &
Potucek (2002)
Teach function 1 participant with autism
6 years old
Aided with no
speech output
Increase (1)
Multiple baseline
*Linton & Singh (1984)
Alternating treatments
Teach two or more
word combinations
2 participants with MR
and hearing impairments
Unaided Increase (2)
18 & 59 years old
Pecyna (1988)
Case study
Teach single words 1 participant with MR
4 years old
Aided with no
speech output
Increase (1)
Remington & Clarke (1993) Teach single words 4 participants with MR Unaided Increase (3)
Alternating treatments 4–11 years old No change (1)
Romski, Sevcik, &
Pate (1988)
Case study
Teach function 3 participants with MR
17–19 years old
Aided with no
speech output
Increase (1)
No change (2)
*Sisson & Barrett (1984)
Alternating treatments
within a multiple
baseline
Teach two or more
word combinations
3 participants with MR and
behavior disorders
4–8 years old
Unaided Increase (3)
Note. All studies, except those marked with an asterisk, lacked the experimental control required to determine reliably the relationship between AAC
intervention as an independent variable and natural speech production as a dependent variable; the evidence presented in these studies is inconclusive.
AAC = augmentative and alternative communication; MR = mental retardation; HI = hearing impairment.
a
Disability is reported per the studies cited.
b
Participants were included if there was some measure of speech production provided before and during or
after AAC intervention. Participants were excluded if they demonstrated ceiling effects for the speech measures at baseline prior to AAC intervention.
c
Data regarding changes in speech production were not reported for individual participants but as a group. The authors concluded that the children
demonstrated increases in speech production.
Table 1 Continued. Studies, published between 1975 and 2003, involving AAC interventions with individuals with developmental disabilities
that documented speech production before and during/after intervention.
Study/design Goal No. of participants and age
a,b
AAC intervention
Authors’ conclusions re.
speech (no. of participants)
Millar et al.: Natural Speech and AAC 253
(b) there was no change in speech for 7 participants
(11%), and (c) speech decreased for 5 participants (7%).
However, many of the studies (17 of 23) did not estab-
lish experimental control with respect to the relationship
between AAC intervention and natural speech production,
making it difficult to draw reliable conclusions about
this relationship from these studies. According to the
certainty of evidence coding scale, these studies were
considered ‘‘inconclusive.’’ Six of the 23 studies used re-
search designs that established experimental control with
respect to the relationship between AAC intervention
and speech production (at least for some of the partici-
pants) and established the reliability of the measures
of speech production as the dependent variable. Unfor-
tunately, none of these studies reported data on treat-
ment integrity. As a result, the evidence that these
studies provided is not ‘‘conclusive.’’ However, these 6
studies provided strong evidence about the effects of AAC
interventions on speech production. In this review, al-
though none of the evidence was conclusive, these 6 stud-
ies used the most rigorous designs from the studies that
met the selection criteria. Therefore, the 6 studies rep-
resent the best evidence available (see Table 2).
Participants
There were a total of 17 participants
1
with devel-
opmental disabilities in the six studies: 4 had autism, and
13 had mental retardation, Down syndrome, or devel-
opmental delay (with 6 of these participants having a
hearing impairment as well). Of the 17 participants, 11
were children and 6 were adults (age range = 2;4–60;0).
Some of the 17 individuals participated in more than
one treatment condition (e.g., introduction of two differ-
ent AAC interventions, such as signs taught through
positive practice compared with signs taught through
positive practice plus positive reinforcement, in an
alternating treatment design). Each individual’s involve-
ment in each treatment was considered as a separate
case, resulting in a total of 27 cases across the six studies.
AAC Intervention
Five of the six studies investigated the effects of
unaided AAC interventions, specifically instruction in
manual signs; the remaining study considered the ef-
fects of aided AAC systems without speech output (i.e.,
Charlop-Christy et al., 2002). The mean length of the
AAC interventions in these studies was 42 sessions, with
a range of 4 to 206 sessions. In the majority of the cases
(78%), the AAC interventions were highly structured and
directed by the clinician; in only 22% of the cases were the
AAC interventions implemented in child-centered play
activities. The studies investigated a range of interven-
tion procedures (e.g., directed rehearsal alone, directed
rehearsal with positive reinforcement, positive practice,
positive practice plus reinforcement).
Outcomes of the AAC Interventions
Of the 27 cases that provided the best evidence, in-
creases in speech production were observed in 89% (24
of 27 cases). In the remaining 3 cases (11%), there was
no change in speech production when baseline data were
compared with data collected during and/or after AAC
intervention. None of the 27 cases showed a decrease in
speech production as a result of AAC intervention.
In 10 of the 27 cases (37%), the PND was at least
90. According to the classification system proposed by
Scruggs et al. (1987), this level of PND suggests that the
AAC interventions were ‘‘highly effective’’ in increasing
speech production in these cases. As Kazdin (1978) ex-
plained, ‘‘If performance during an intervention phase does
not overlap with performance during the baseline phase
when these data points are plotted over time, the effects
usually are regarded as reliable. The replication of non-
overlapping distributions during different treatment
phases strongly argues for the effects of treatment’’ (p. 637).
Most of the gains in natural speech in the 6 studies
were measured in terms of the number of spoken words
or word approximations produced by the participants
(12 of the 24 cases where gains were observed); the mean
gain across the AAC applications in these cases was 13
words (range = 1–52 words). In 6 of the 24 cases where
gains were observed, speech production was reported
in terms of the number of spoken two-word phrases
produced by the participants; the mean gain across the
AAC applications in these cases was 6 two-word phrases
(range = 4–7 two-word phrases). In 6 cases, speech pro-
duction was reported as the percentage of opportunities
in which the participant produced words or word ap-
proximations; the mean gain was 77%, with a range of
40% to 100%. Ceiling effects were observed for 17 of the
27 cases in the measures of speech production after AAC
intervention; these ceiling effects may have limited the
gains observed.
In 79% of the cases where there were gains in speech
production (19 of 24 cases), these gains were observed
shortly after the AAC intervention was initiated (within
1
Two of the participants in the study by Conaghan et al. (1992) and 2 of the
participants from the study by Linton and Singh (1984) were excluded from the
best evidence analysis because these participants demonstrated ceiling effects
on the measures of speech production at baseline in both treatment conditions,
leaving no room for gains on the speech production measures. One participant
in the Linton and Singh study (J.) demonstrated ceiling effects for speech
measures in one treatment (T1: signs taught throughpositive practice) but not
in the other treatment (T2: signs taught through positive practice plus
reinforcement); the former case was omitted from the analysis but the latter
was included. Two participants from the study by Kouri (1988) were also
excluded from the best evidence analysis because data were not reported from
repeated measures of speech production within the withdrawal design for these
participants.
254 Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006
Table 2. Participant information, AAC interventions, and speech outcomes for studies presenting the ‘‘best evidence’ regarding the effects of AAC intervention on the speech production of
individuals with developmental disabilities.
Participant AAC intervention Speech outcomes
Study/design
Participant ID
and gender Disability
a
Age Type of AAC
Treatment
condition
b
No. of
sessions
c
PND
Change in speech production
during/after AAC intervention
d
Barrett & Sisson (1987)
Alternating treatments design
within a multiple-
baseline design
J.
Male
Mod. MR;
behavior disorder
5;3 Unaided
manual signs
T1 (TC)
T2 (MTC)
87 (2)
87 (2)
57
51
Increase; 3 words*, 1 word*
Increase; 2 words, 1 word
M.
Male
Mod. MR;
behavior disorder
13 T1(TC)
T2 (MTC)
67 (2)
67 (2)
38
72
Increase; 2 words, 1 word
Increase; 3 words*, 2 words
Charlop-Christy et al. (2002)
Multiple baseline
A.
Male
Autism 12 Aided with
no speech
output
T1 (PECS-
Academic)
T2 (PECS-Play)
4
4
67
33
Increase; 60% of opportunities*
e
Increase; 40% of opportunities*
J.
Male
Autism 3;8 T1 (PECS-
Academic)
5 82 Increase; 100% of opportunities*
T2 (PECS-Play) 5 73 Increase; 80% of opportunities
K.
Male
Autism 5;9 T1 (PECS-
Academic)
9 43 Increase; 80% of opportunities*
T2 (PECS-Play) 9 50 Increase; 100% of opportunities*
Conaghan et al. (1992)
Alternating treatments design
J.
Male
Profound MR;
mod. bilateral HI
18 Unaided
manual signs
T1 (DR)
T2 (DR + PR)
27
56
93
98
Increase; 6 two-word phrases*
Increase; 7 two-word phrases*
T.
Male
Severe MR;
mild bilateral HI
36 T1 (DR)
T2 (DR + PR)
23
39
0
0
No change
No change
F.
Male
Profound MR; bilateral
high-freq. HI
60 T1 (DR)
T2 (DR + PR)
16
31
94
94
Increase; 7 two-word phrases*
Increase; 7 two-word phrases*
M.
Female
Severe MR; mild HI in
left, severe HI in
right
18 T1 (DR)
T2 (DR + PR)
11
25
100
100
Increase; 4 two-word phrases
Increase; 4 two-word phrases*
Kouri (1988) J.S. Autism 3 Unaided T1 (TC) 24 (2) 21 Increase; 0 words, 10 words
Withdrawal design Male manual signs
(Continued on the following page)
Millar et al.: Natural Speech and AAC 255
Table 2 Continued. Participant information, AAC interventions, and speech outcomes for studies presenting the ‘‘best evidence’ regarding the effects of AAC intervention on the speech
production of individuals with developmental disabilities.
Participant AAC intervention Speech outcomes
Study/design Participant ID
and gender Disability
a
Age Type of AAC
Treatment
condition
b
No. of
sessions
c
PND
Change in speech production
during/after AAC intervention
d
T.A. Dev. delay 2;4 T1 (TC) 17 (2) 45 Increase; 20 words, 20 words
Male
B.V. Down syndrome 2;10 T1 (TC) 25 (2) 82 Increase; 5 words, 52 words
Female
Linton & Singh (1984)
Alternating treatments
design
J.
Male
Profound MR;
mod. bilateral HI
18 Unaided
manual signs
T2 (PP + R) 30 90 Increase; 2 words*
F.
Male
Severe MR; bilateral
high-freq. HI
59 T1 (PP)
T2 (PP + R)
9
19
0
63
No change
Increase; 1 word*
Sisson & Barrett (1984)
Alternating treatments
design within a
multiple-baseline
design
E.
Male
Mod. MR;
Behavior disorder
7 Unaided
manual signs
T1 (TC) 127 (3) 100 Increase; 4 words*, 2 words*,
2 words*
M.
Male
Mild MR;
Behavior disorder
8;1 T1 (TC) 206 (3) 100 Increase; 4 words*, 4 words*,
4 words*
T.
Male
Mild MR;
Behavior disorder
4;8 T1 (TC) 113 (3) 98 Increase; 4 words*, 3 words*,
3 words*
Note. PND = percentage of nonoverlapping data; mod. = moderate; MR = mental retardation; TC = total communication, MTC = modified total communication, PECS = picture exchange
communication system; HI = hearing impairment; DR = directed rehearsal; DR + PR = directed rehearsal plus positive reinforcement; PP = positive practice; PP + R = positive practice plus reinforcement;
DDdev. = developmental; freq. = frequency.
a
Disability is reported per the studies cited.
b
The different intervention conditions in each study are designated by different numerals (e.g., in Conaghan et al., 1992, the treatment consisted of instruction in manual
signs using directed rehearsal alone [Treatment 1 = T1] and in combination with positive reinforcement [Treatment 2=T2]).Thetypesof intervention are coded as described in the studies.
c
The total number of
sessions is reported. If the sessions occurred in two or three sets (as in a multiple baseline across two or three sets of stimuli), the number of sets of sessions is indicated in parentheses (e.g., in the Barrett & Sisson study,
there were a total of 87 sessions for J. in T1, grouped in two sets: one set of 75 sessions that targeted the first set of sentences and one set of 12 sessions that targeted the second set of sentences).
d
The gain scores
represent approximate values because they were extrapolated from the graphic presentation of the data in the studies. Gain scores are reported separately for each application of the AAC intervention when the
intervention was applied to more than one set of stimuli, as in a multiple-baseline-across-behaviors design, or when the treatment was replicated, asinanABABdesign.Gainscoresaremarkedbyanasteriskifthere
were ceiling effects. For example, in the Barrett & Sisson study, in T1 (TC), J. demonstrated an increase of 3 spoken words when the AAC intervention was applied to the first set of sentences and a gain of 1
spoken word when the AAC intervention was applied to the second set of sentences; ceiling effects were observed for both sets of sentences.
e
The percentage of structured opportunities with spontaneous speech
was measured in each session with the participants. Gain scores are reported as a change in the percentage of opportunities with spontaneous speech, comparing the maximum point in baseline with the maximum
point following AAC intervention.
256 Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006
the first 5 sessions). In the remaining 5 cases (21%), there
was a lag between the initiation of AAC intervention and
the observation of increases in speech production, rang-
ing from 6 sessions to 25 or more sessions.
Discussion
The best evidence indicates that AAC interventions
do not have a negative impact on speech production. All
but 1 of the 17 participants (94%) in our review dem-
onstrated increased speech production during or follow-
ing at least one of the AAC interventions investigated.
The remaining participant, a 36-year-old man with se-
vere mental retardation and a mild bilateral hearing
impairment (‘‘T.’’ in the study by Conaghan et al., 1992),
showed no change in speech production in either of the
two AAC interventions in which he participated (i.e.,
signs taught through directed rehearsal and signs taught
through directed rehearsal plus positive reinforcement).
The positive effects of AAC intervention on speech
production were observed across children and adults,
ranging in age from 2 years to 60 years. These results are
encouraging and suggest that speech gains may still be
realized by individuals with developmental disabilities
well past the critical early childhood years. Positive
effectswerealsoobservedacrossarangeofAAC
intervention approaches, including highly structured,
clinician-directed instruction grounded in behavioral
theory and child-centered approaches implemented in
play contexts. The facilitative effects on speech produc-
tion were robust across different instructional ap-
proaches. The majority of the best evidence studies
investigated the effects of unaided AAC (five of the six
studies); only one study included aided AAC systems
(without speech output). Future research is required to
investigate the effects of aided AAC systems on speech
production for children and adults with developmental
disabilities. Given the limited number of participants in
the studies, the wide variation in participant character-
istics and AAC interventions, and the range of speech
measures used, it is not possible to generalize the find-
ings of the studies to the population of individuals who
require AAC. Considerably more research is required to
determine what factors may best predict gains in speech
production as a result of AAC interventions.
Most of the speech gains observed were modest ones
when considered in absolute terms. The mean gain for the
studies that provided the best evidence was an increase of
13 words (range = 1–52 words) and an increase of 6 spoken
two-word phrases (range = 4–7 two-word phrases). It is
important to note that AAC interventions are typically
implemented to build communication and language skills
through a range of modalities (including signs and aided
AAC systems as well as natural speech), rather than to
increase speech production alone. To date, there is limited
evidence to determine whether AAC interventions are the
most effective means to increase speech production if this
is the primary goal of intervention. Future research is
needed to determine the comparative effectiveness and
efficiency of interventions to promote speech production.
There is evidence, however, to support the conclusion that
AAC enhances communicative competence and language
skills (Light et al., 1999) and has at least modest benefits
for increases in speech production.
There are a number of possible factors that may
account for the modest gains in speech production that
were observed: (a) the characteristics of the participants
(12 of the 17 participants were 5 years old or older, 6 had
hearing impairments, and many had significant cogni-
tive impairments); (b) the relatively short duration of the
AAC interventions (mean of 42 sessions); (c) the limited
corpus of words targeted in intervention; and/or (d) the
ceiling effects observed in the speech measures after AAC
intervention.
Although the speech gains were modest ones when
considered in absolute terms, these gains should also be
considered in relative terms, specifically in the context of
the total number of words introduced through the AAC
interventions as well as in the context of the number of
words in the participants’ repertoires prior to the AAC
interventions. In four of the six studies, the data on
speech production were collected exclusively within
structured naming tasks; in these studies, only a limited
number of words were introduced and gain scores were
constrained by the number of trials (words or phrases
targeted). Ceiling effects were observed in the speech
production data collected during or after the AAC in-
tervention in 17 of the 27 cases; in these cases, the par-
ticipants learned 100% of the spoken words or phrases
that were introduced through AAC intervention. When
considered in this context, the gain scores are impressive.
Given the ceiling effects observed, the gains reported
might underrepresent the effects of AAC intervention on
speech production. In fact, the largest gains in speech
production were recorded in the study by Kouri (1988), in
which data on speech production were collected during
free-play interactions and were, thus, unconstrained by
ceiling effects. In the 3 cases in this study, the gains in
speech production after AAC intervention ranged from
10 words to greater than 50 words.
The speech gains should also be considered in light of
the numberof words in the repertoires of the participants
at baseline prior to AAC intervention (Schlosser, 2003).
For example, Kouri (1988) reported that the 3 partic-
ipants in her study (J.S., T.A., and B.V.) had very limited
speech repertoires prior to AAC intervention (0, 3–5, and
8 spoken words, respectively); their repertoires were 4 to
10 times greater after the AAC interventions (gains of 10,
20, and 52 spoken words, respectively). These gains
Millar et al.: Natural Speech and AAC 257
are impressive, particularly given the relatively short
duration of the AAC intervention (17–25 sessions).
Unfortunately, most of the studies provide limited data
on the participants’ speech repertoires preintervention,
limiting this type of analysis.
The best evidence analysis also suggests that, in
some cases, the initial introduction of an AAC interven-
tion may not have resulted in immediate gains in speech
production; in 21% of the cases, there was a lag between
the onset of AAC intervention and evidence of gains in
speech production. It has been argued that AAC inter-
vention may allow individuals with developmental dis-
abilities to bypass the motor and cognitive demands of
speech production and focus on building communication
and language skills instead; after individuals establish
basic communication and language skills, they may be bet-
terabletoreallocateresourcestoimprovetheirspeech
productions (Romski & Sevcik, 1996). This theory would
predict a delay in speech gains following the initiation of
AAC intervention, as was observed for some of the cases
in the best evidence review. The lag in speech gains may
also be explained by the initial learning demands im-
posed by the AAC systems. It may be that some indi-
viduals with developmental disabilities must focus on
mastering the operational demands of AAC during the
early stages of AAC intervention (e.g., producing the cor-
rect hand shape, position, orientation, and movement of
the sign; selecting the correct graphic symbol; Blischak,
2003; Light, 1989, 2003); speech gains may only be realized
after these operational skills are learned and resources
can be reallocated to speech production. This latency ef-
fect is important to note because it causes an overlap in
the data in baseline and treatment phases, decreasing the
level of PND. The effectiveness of the AAC intervention
in increasing speech production may be underestimated.
For most of the participants in the best evidence
analysis, however, the gains in speech production were
observed shortly after the introduction of the AAC inter-
vention. These cases argue against resource allocation as
a theoretical explanation of the facilitative effects of
AAC. Instead, the data in these cases better support the
theory of automatic reinforcement, which suggests that if
AAC is presented along with speech and followed by a
reinforcer, both AAC and natural speech should increase
in frequency. Future research is needed to better de-
lineate the theoretical mechanism(s) that account for the
facilitative effects of AAC on speech production for in-
dividuals with developmental delay.
Implications for Practice
The results of the best evidence analysis should
assuage the fears of parents and professionals about the
potential negative impact of AAC intervention on speech
production. Clinicians and parents should not hesitate to
introduce AAC interventions to individuals with devel-
opmental disabilities whose speech is inadequate to meet
their communication needs. AAC intervention has sig-
nificant benefits in the development of communicative
competence and language skills; the present best evi-
dence analysis provides data that suggest AAC inter-
ventions can also have positive benefits for natural
speech production. Too often, AAC is pursued as a last
resort with individuals with developmental disabilities
for fear that AAC will inhibit the development of speech.
Frequently, clinicians perceive that they must make an
‘‘either– or’’ decision: either pursue the development of
natural speech or introduce AAC (Beukelman, 1987).
This review provides empirical evidence to support the
counterargument that AAC intervention facilitates the
production of natural speech. This evidence, coupled with
the existing evidence that AAC interventions support
the development of communicative competence and lan-
guage skills, provides a strong case for implementing
AAC with individuals with developmental disabilities who
are unable to meet their communication needs through
natural speech.
Results of the best evidence analysis also suggest
that clinicians and parents should not be concerned if
increases in speech production do not occur immediately
after initiation of the AAC intervention; in 21% of the
cases reviewed, speech gains were observed after a lag of
6–25 sessions. In keeping with evidence-based practices
and outcomes measurement, clinicians should carefully
monitor the effectiveness of AAC interventions with indi-
vidual clients to determine the effects on communicative
competence, social interaction, language skills, and
speech production.
Limitations of the Research
The results of this best evidence analysis provide
important preliminary empirically based evidence to
guide parents and professionals in decision making re-
garding AAC interventions. However, there are a number
of limitations to this best evidence analysis that must be
considered in interpreting and applying the results.
None of the studies had the primary goal of de-
termining the impact of AAC on speech development. As
a result, some of the studies (outside the best evidence
analysis) did not establish experimental control with re-
spect to these variables and failed to establish the re-
liability of the speech measures and treatment integrity.
It was not possible to draw reliable conclusions about the
impact of AAC on natural speech for some of the studies;
the certainty of evidence was inconclusive.
Six of the 23 studies had sufficient methodological
rigor to support conclusions regarding the effect of AAC
on speech production. These studies involved a total of
258 Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006
17 participants, with a total of 27 treatment cases. Al-
most all of the participants in the studies had mental
retardation, developmental delays, or autism; hence, they
represented only one subset of the diverse population of
individuals who require AAC. Furthermore, variation
in the participants’ speech production and intelligibility
was not clearly defined in most studies. These limita-
tions make it difficult to generalize results to others who
require AAC.
The current review included studies published be-
tween 1975 and 2003; however, only 11 of the 23 studies
that met the criteria for inclusion were published since
1990. Furthermore, only 2 of the 6 studies included in the
best evidence analysis were published after 1990. There
have been significant advances in AAC systems and prac-
tices since this time. Some of the studies included in the
review used intervention approaches that would not be
considered current best practices. Speech outcomes may
differ when AAC interventions that use best practices are
implemented.
Most of the studies in the best evidence analysis im-
plemented unaided AAC systems (i.e., manual signs). Re-
sults may not generalize to interventions implementing
aided systems (with speech output) because these systems
impose different learning demands. Although all of the
studies provided AAC interventions, they used different
instructional procedures to teach AAC. Of the studies that
provided the best evidence, only two investigated AAC
that was taught through child-centered intervention in a
play context. The other four studies used highly struc-
tured instruction that was based on behavioral theory;
each of these four studies used different types of instruc-
tional procedures (e.g., positive practice alone or in com-
bination with positive reinforcement [Linton & Singh,
1984], directed rehearsal with and without positive re-
inforcement [Conaghan et al., 1992]). Given the small
number of studies analyzed and the diversity of instruc-
tional techniques used, it is not possible to draw precise
conclusions regarding the relative effectiveness or effi-
ciency of various approaches to instruction in AAC. Some
instructional methods may be more advantageous for
specific populations in enhancing communicative compe-
tence, language development, and speech production.
The speech gains reported in the best evidence anal-
ysis were based on visual inspection of the graphs pro-
vided in the studies and manyof these graphs were small
in scale. As a result, it was necessary to estimate the
values of specific data points. Furthermore, in more than
half the cases, ceiling effects were observed in the speech
measures, making it difficult to draw reliable conclusions
about the extent of the impact of AAC intervention on
speech production. Most of the studies only collected data
for a limited corpus of words or phrases within structured
naming tasks; only the study by Kouri (1988) collected
data within communicative interactions in play contexts
but these were conducted within a clinic setting. None of
the studies investigated the generalization of speech
gains to functional use in the natural environment.
Future Directions
Future research is urgently needed to more clearly
delineate the relationship between AAC intervention and
natural speech production. To establish the certainty of
the evidence, this research should be designed with suf-
ficient methodological rigor to establish experimental con-
trol, ensure the reliability of the dependent measures of
speech production, and ensure the integrity of the AAC
interventions. Furthermore, the studies should clearly
document participant characteristics (e.g., cognition, lan-
guage comprehension, speech production, intelligibility)
and AAC intervention procedures. Systematic research is
needed to determine participant characteristics that may
positively or negatively influence the development of natu-
ral speech; for example, individuals with severe hearing
impairments or cognitive deficits may be limited in the
development of speech. Specifically, research studies are
needed (a) to investigate the effects of various types of
AAC systems (i.e., unaided systems, nonelectronic aided
systems, electronic systems with speech output) and
various instructional approaches on speech production,
(b) to determine the impact on individuals of various ages
with a range of developmental disabilities, (c) to identify
factors that may influence the effects of AAC intervention
on speech production, (d) to evaluate the extent of gen-
eralization of speech gains to functional use in real-world
situations, and (e) to document the long-term effects of
AAC on speech production over an extended time period.
AAC offers significant benefits for individuals with
developmental disabilities in terms of enhancing com-
municative competence and promoting language devel-
opment. The present research review provides important
preliminary evidence that AAC interventions do not in-
hibit speech production; instead, AAC may also support
speech production.
Acknowledgments
We would like to thank the graduate students at The
Pennsylvania State University and Northeastern University
who contributed to the project. Portions of this research were
presented at the Biennial Conference of the International
Society for Augmentative and Alternative Communication,
Washington, DC, August 2000.
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Received March 3, 2004
Revision received April 21, 2005
Accepted August 25, 2005
DOI: 10.1044/1092-4388(2006/021)
Contact author: Diane C. Millar, who is now at the Depart-
ment of Communication Sciences and Disorders, Radford
University, P.O. Box 6961, Radford, VA 24142.
E-mail: dcmillar@radford.edu
Millar et al.: Natural Speech and AAC 261
Appendix A. Journals searched.
Journal name
American Association for the Education of the Severely/Profoundly Handicapped
American Journal of Mental Deficiency
American Journal of Speech-Language Pathology
American Journal of Mental Retardation
Analysis and Intervention in Developmental Disabilities
Aphasiology
Applied Psycholinguistics
Applied Research in Mental Retardation
Assistive Technology
Augmentative and Alternative Communication
Australia and New Zealand Journal of Developmental Disabilities
Australia Journal of Human Communication Disorders
Behavior Modification
British Journal of Disorders of Communication
British Journal of Developmental Disabilities
British Journal of Mental Subnormality
Education and Training in Mental Retardation
Education and Training of the Mentally Retarded
Education and Treatment of Children
European Journal of Communication
European Journal of Disorders of Communication
Exceptional Children
Exceptionality
International Journal of Language and Communication Disorders
Journal of Applied Behavior Analysis
Journal of the Association for Persons with Severe Handicaps
Journal of the Association for the Severely Handicapped
Journal of Autism and Childhood Schizophrenia
Journal of Autism and Developmental Disorders
Journal of Behavioral Education
Journal of Childhood Communication Disorders
Journal of Communication Disorders
Journal of Experimental Child Psychology
Journal of Intellectual Disability Research
Journal of Mental Deficiency Research
Journal of Psycholinguistic Research
Journal of Special Education
Journal of Special Education Technology
Journal of Speech and Hearing Disorders
Journal of Speech and Hearing Research
Journal of Speech, Language, and Hearing Research
Language, Speech, and Hearing Services in Schools
Mental Handicap Research
Mental Retardation
Remedial and Special Education
Research in Developmental Disabilities
Sign Language Studies
Topics in Early Childhood Special Education
262 Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006
Appendix B. Operational definitions for coding the studies.
Coding category Operational definition
Study identification Record the authors of the study and the year that the study was published.
Goal of the study Record the goals of the study according to the following categories: (a) teach single-word vocabulary; (b) teach two or more
word combinations; (c) teach specific communicative function or intent (e.g., request, comment); (d) other (i.e., the goal
of the study was other than those listed).
Design of the study For single-participant designs, record the design of the study according to the descriptions by Barlow and Hersen (1984)
and Kearns (1986): (a) single-participant, alternating treatments design; (b) single-participant, multiple-probe or
multiple-baseline design (across participants, behaviors, stimuli, or settings); (c) single-participant withdrawal design; (d)
descriptive case study; (e) other (i.e., a design other than those listed; if so, specify the design). For group designs, record
the design of the study according to the descriptions by Ventry and Schiavetti (1986): (a) one group pretest–posttest;
(b) randomized pretest–posttest control group; (c) Solomon randomized four-group design; (d) static-group comparison;
(e) nonequivalent control group; (f) time series; (g) other (i.e., a design other than those listed; if so, specify the design).
Participants
Identification Record the letter(s) or numbers used in the study to identify each participant.
Gender Record the participant’s gender as reported by the authors.
Disability Record the primary disability reported for each participant separately, using the terminology of the authors: mental
retardation, autism, developmental delay, cerebral palsy, developmental apraxia, or other. List any associated
impairments listed by the authors: hearing impairment, visual impairment, behavior disorder.
Age Record the chronological age of each participant in years (and months) as reported by the authors.
AAC intervention
AAC systems Record all of the types of AAC system(s) used in the intervention according to the following categories: (a) unaided AAC
systems (i.e., communication modes that use only the communicator’s body, such as manual signs); (b) aided AAC
systems with speech output (i.e., electronic AAC systems that produce either digitized or synthesized speech output);
and/or (c) aided AAC systems without speech output (i.e., nonelectronic AAC systems such as communication boards,
communication books, or other forms of graphic symbols).
Treatment condition Record the treatment condition as described by the authors. Code if the intervention involves: (a) structured,
clinician-directed trials; (b) client-centered play or other daily activities; (c) other (if so, specify).
Sessions Record the number of sessions as reported by the authors orillustrated on the graphs provided. If the sessions occurred in two or
three sets (asin a multiple baselineacross two or three sets of stimuli), the number of sets of sessions is indicated in parentheses.
Outcomes
Effect of AAC Calculate the effect of AAC intervention on speech production using the data provided in the study.
intervention For single-participant experimental designs, report the percentage of nonoverlapping data (PND; Scruggs et al., 1987).
Calculate the PND by dividing the total number of data points in intervention that do not overlap the data points in
baseline by the total number of data points in intervention and multiplying by 100. Code the PND for each participant in
each single-participant design separately according to the procedures established by Schlosser and Lee (2000). If
participants were involved in more than one treatment (as in an alternating treatments design), code the PND for the
individual treatments as separate cases for each participant. When participants were involved in more than one
application of the same AAC intervention (e.g., as in a multiple baseline across behaviors design where the same AAC
intervention was applied across several sets of stimuli or in an ABAB withdrawal design), the researchers reported the
mean PND of the AAC intervention across the applications according to the procedures suggested by Scruggs et al.
(1987). Do not calculate PND in an alternating treatment design when the ‘‘more effective’’ treatment is applied to the
‘less effective’ treatment condition in a C phase because the effects of AAC on speech production cannot be reliably
determined for this C phase because of the confounding effects of the intervening, less effective treatment condition.
For group designs, calculate the effect size. This is determined by calculating the difference between the two group means
(the control group and the experimental group) and dividing by the control group’s standard deviation (Kazdin, 2003).
Report ‘not possible to calculate’ if there are not sufficient data available to calculate the effect of AAC intervention on
speech production.
Changes in speech
production
Calculate the increases, decreases, or lack of change in speech production based on the data provided in the study using the
following formula: subtract the maximum point at baseline (prior to AAC intervention) from the maximum point during or
following AAC intervention. Quantify the change in speech production using the unit of measure observed in each study
(e.g., 5 spoken words). Report the changes in speech production separately for each application of an AAC intervention.
Indicate that it is not possible to calculate changes in speech production reliably if the study is so flawed methodologically
that the evidence is inconclusive.
Millar et al.: Natural Speech and AAC 263
Certainty of evidence Code each study as to the certainty of evidence that it provides with respect to the relationship between AAC intervention(as
the independent variable) and speech production (as a dependent variable). Use the following coding categories based
on N. L. Smith (1991) and Simeonsson & Bailey (1995): (a) conclusive evidence (i.e., the study has a strong design with
adequate or better procedural reliability and reliability of the speech measures; the speech outcomes observed are
undoubtedly the result of the AAC intervention); (b) preponderant evidence (i.e., the study has only minor flaws in the
design with adequate or better procedural reliability and reliability of the speech measures; speech outcomes are not
only possible but they are more likely to have occurred as a result of the AAC intervention than not); (c) suggestive
evidence (i.e., the study has minor design flaws and inadequate procedural reliability or inadequate reliability of the
speech measures; the speech outcomes are plausible as the result of the AAC intervention); or (d) inconclusive evidence
(i.e., the study exhibits major design flaws and fails to establish experimental control; the study’s flaws preclude any
conclusions that speech outcomes are the result of AAC intervention).
Appendix B (p. 2 of 3). Operational definitions for coding the studies.
Coding category Operational definition
264 Journal of Speech, Language, and Hearing Research Vol. 49 248–264 April 2006
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