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Highlighting - Or Why Google Is That Successful.

Highlighting – Or Why Google Is That Successful
Michael D. Kickmeier-Rust & Dietrich Albert
Department of Psychology
University of Graz, Austria
In today’s information society, searching for information or for functions of a software
application is one of the most frequent tasks. Still, visual search tasks are reported to be
associated with low efficiency and user satisfaction. A simple method to improve search
performance is highlighting. With the current experiment we investigate if search
performance and user satisfaction can be significantly improved by presenting an optimal
density of highlighted items. As the results of the eye-tracking experiment yielded, presenting
15 percent of highlighted items on a display improved search speed, accuracy, as well as user
satisfaction in contrast to presenting no highlighted items or 49 percent. The results are
discussed regarding their practical impact, design implications, and regarding emerging
questions for future research.
Keywords: Highlighting, Search Performance, Visual Search, Visual Attention, Eye-tracking
1. Introduction
The basics of human visual perception and attention are a major foundation of software usability.
For instance in information design, legibility, brightness, contrast, color, saliency, or the structure
of information are crucial factors to enable successful and efficient information retrieval.
Searching (e.g., a piece of text on a website or a certain menu entry of a software
application) is probably the most common tasks of human-computer interaction. Efficiency,
accuracy, and user satisfaction of that interaction are strongly influenced by the visual properties
and the amount of visual stimuli of a display. We are facing a vast amount of electronically
available information, software applications providing countless functions, and searching is
reported to be oftentimes difficult, inefficient, and unsatisfying [2, 7]. The infinite number of
stored bits and the variety of software functions, however, are only valuable, if they can be
searched, found, and used by humans.
The question arises whether information or functionality can be displayed in a way that
performance, accuracy, and user satisfaction of visual search processes are possibly high; even
without knowing what someone is searching for.
Kickmeier-Rust, M. D., & Albert, D. (2005). Highlighting – Or why Google is that
successful. In A. Holzinger & K. H. Weidmann (Eds.), Empowering software quality:
How can usability engineering reach these goals? (pp. 45-52). Wien: Österreichische
Computer Gesellschaft.
A well-tried and successful method to facilitate visual search is highlighting; on paper as
well as on electronic displays. There is a variety of research concerning the basic features of
highlighting [14] and applied techniques of highlighting [8].
Highlighting is based on early visual processes. In a pre-attentive stage of vision the
visual field can be processed as a whole without limitations of capacity. For example, if one
searches a red circle among green circles, it can be detected immediately no matter how many
green circles surround the red target. In terms of Ulric Neisser [6], the red circle pops out of the
display. Generally, such pop-out effect occurs if the target is defined by single basic feature such
as color, shape, size, or motion [14]. According to the Guided Search model [13, 15, 16], these
early visual processes create an activation map. An activation map is a representation of the
visual field determined by continuous levels of activation. The activation for a certain location in
the visual field is determined by the visual properties of the item(s) in that location and
intentional control. For example, when someone is searching for a target, which is expected to be
red, a single red circle among blue ones provokes higher activation for its location than a single
green circle. In a later stage of vision, attention is directed towards the peaks of the activation
map. This guidance is determined by bottom-up aspects (e.g., saliency) and top-down aspects
(e.g., a searchers intentions or expectations). Attention acts like a filter for relevant information to
be passed to one or more limited-capacity processes of vision; only locations that are promising
for a certain goal are attentively processed.
So far, visual attention refers to focal fixations, e.g. fixating a letter. Information
processing, however, does not only occur for focally attended stimuli but also for the periphery of
a stimulus. While the fovea is amounting for 2 degree visual angle, which corresponds to about 2
per mille of the visual field, peripheral vision allows the processing of information from the
whole visual field. Due to the decrease of neurons related to retinal areas with increase
eccentricity, however, visual performance generally decreases with eccentricity [13]. Still, there
is evidence for the importance of peripheral vision in search tasks. Besides pre-attentive
processing of the whole visual field, peripheral vision contributes to feature detection [1] or letter
recognition [12] during search episodes.
Past research on highlighting, primarily focused on valid highlighting. Fisher and
colleagues [3, 4] argued that a key variable of facilitating visual search through highlighting is
the frequency a target, as opposed to a distractor, is highlighted. The aim of the current
experiment is to investigate the effects of the density of highlighted items in a display.
On the basis of mechanisms of visual search, as introduced very briefly, we assume that
presenting an optimal density of highlighted items increases visual search performance. The
distribution of highlighted items facilitates the creation of an activation map in a pre-attentive
stage of vision and, subsequently, guides the gaze through the display. Due to mechanisms like
the inhibition of return [10] we can assume that these are attended only once along a serial path.
A target can be detected either if itself is highlighted or, by means of peripheral information
processing, if it surrounds a fixated (highlighted) item in a certain distance. This guidance is
important because the effort a searcher spends on surveying a display is limited by means of a
cost-benefit analysis, as outlined in the Information foraging Theory [9]. Not all objects within an
area to search are attended during a search episode. When people are searching information they
roughly scan through the visual field. However, if pre-attentive guidance corresponds to these
limitations, the full search capacity can be utilized for target detection.
With the current eye-tracking experiment we investigate the impact of the total density of
highlighted items in a display on search performance, accuracy, and user satisfaction.
2. An Eye-Tracking Experiment
2.1. Experimental Design
In an eye-tracking experiment participants were presented 48 panels consisting of 140 simple
nouns each. The task was to determine as fast and as accurate as possible, whether a given search
word was present among the nouns of the panel or not. In 26 tasks the target was present and in
22 tasks the target was absent. On the basis of a previous study on density effects of highlighting
[5] which reported highest search performance between 15 and 30 percent highlighting, three
different densities were realized: no highlighted items (0 percent) as a too small portion of
highlighting, 15 percent, which was supposed to be an optimal density, and 49 percent as too
high portion of highlighting. We realized 49 percent in order to avoid an inflection point at about
50 percent when the number of highlighted items starts to exceed the number of non-highlighted
items and non-highlighted items start becoming more salient than highlighted ones. The sequence
of tasks and the distributions of words and highlighting were created randomly for each
participant. As dependent variables response times, answer accuracy (correct and incorrect
responses), and eye movements were recorded.
2.2. Participants
The data of 36 participants were analyzed, thereof 27 females (75 percent) and 9 males (25
percent). The average age was 22.53 years (SD = 3.14), the youngest participant was 19 years of
age, the oldest 31. In total, 21 participants reported normal vision, 15 reported vision corrected to
normal. One participant of the “corrected to normal” group additionally reported a minor color
vision defect.
2.3. Material and Apparatus
We used the SMI Eyelink I eye-tracker (SensoMotoric Instruments GmbH, Germany; to record binocular eye-movements. The eye-tracking experiment was developed
using Microsoft Visual C++ 6.0 and the SMI Eyelink API templates. The test software presented
introduction and briefing screens and, subsequently, generated automatically random search
panels from a pool of simple nouns. To counter-balance position effects, the different search
tasks were automatically presented in random order for each participant. The experimental
sequence was presented on a 21” computer monitor with a screen resolution of 1280x1024. At a
distance of 58 cm from the screen a chin rest was mounted to stabilize head movements and the
distance between eyes and screen. Responses were given using the Y key (yes, the target is
present) and the N key (no, the target is absent) on a regular computer keyboard with German
layout. The keys were prepared with stickers that allowed haptic detection in order to prevent
participants from looking down to the keyboard during the experimental tasks.
2.4. Procedure
Participants were first asked to conveniently take place in front of the eye-tracker’s subject
monitor. Then the participants were briefed regarding the aims of the experiment and the eye-
tracking procedure. In a next step a paper-pencil data form was filled out, asking for age, sex,
education, vision (normal, corrected to normal, vision below 100 percent, and vision defects), and
Internet and search engine usage. Then participants were instructed to determine as fast and as
correct as possible whether a given word was present in the display or not. Additionally, the
participants were briefed to inform the experimenter about any events during search tasks, e.g.
forgetting the target word or usage of the wrong response key. After initial briefings the eye-
tracker’s head-mounted camera system was applied carefully and eye cameras were adjusted.
After dousing lights, eye cameras were focused on the pupils and cameras were calibrated and
validated. The camera adjustment, calibrations, and validations required about 10 to 15 minutes.
The experiment started with three training tasks. After successful completion of the
training, participants performed the first block of 24 search tasks. Each task was initiated with the
fixation of a center point on the screen. During this fixation the participants were told the target
word. The participants started the search task by pressing the space key. To terminate a search
task participants pressed either the Y key (yes, the target is present) or the N key (no, the target is
After the first block of 24 search tasks a ten minutes break followed. During this break the
uncomfortable head-mounted camera system was removed. The second block of search tasks was
performed identical to the first one. Finally, the camera system was removed and participants
rated three questions regarding their satisfaction with the different densities of highlighting.
Arising comments of participants were note by the experimenter.
3. Results
3.1. Response Times
As hypothesized, we found significantly shorter response times when presenting 15 percent
highlighted items in the search panel, in contrast to no highlighted items or 49 percent. In target-
present tasks mean response time for the 15-percent group was 12.62 seconds (SD = 7.25), in the
0-percent group 14.85 seconds (SD = 5.75), and in the 49-percent group 15.69 seconds
(SD = 7.35). In target-absent tasks mean response time for the 15-percent group was 23.6 seconds
(SD = 6.33), in the 0-percent group 26.01 seconds (SD = 7.84), and in the 49-percent group 23.23
seconds (SD = 6.70). As shown in Figure 1a, in total the 15-percent group yielded a mean
response time of 18.11 seconds (SD = 6.79), the 49-percent group 19.46 seconds (SD = 7.03),
and the 0-percent group 20.43 seconds (SD = 6.80). An analysis of variance (ANOVA) reported a
significant effect of the density of highlighted items (F(2, 1132) = 2.28, p = .013).
3.2. Accuracy
Focusing on target-present tasks only, also for answer accuracy (relative frequencies of correct
responses) we found expected results. Answer accuracy was significantly higher when presenting
15 percent highlighted items in the search panel, in contrast to no highlighted items or 49 percent.
As shown in Figure 1b, presenting 15 percent of highlighted items resulted in a relative frequency
of correct responses of .79; for the 0-percent group we found a relative frequency of .67 and in
the in the 49-percent group a relative frequency of .65. An ANOVA reported a significant
difference between the three highlighting densities (F(2, 1132) = 43.71, p < .001.).
(a) (b)
0% 15% 4 9%
De nsity of highlig hte d item s
Re spo ns e time s (s)
0% 15% 49 %
De nsity of h ighlighte d item s
Re lative frequ . of corre ct res po ns es
Figure 1. Panel (a) shows the response times for different densities of highlighted
items in the search panel; panel (b) the relative frequencies of correct responses.
3.3. Scan Paths
Analysis of eye movements yielded that the gaze was immediately shifted from the initial center
fixation to the item in top left corner of the search panel. This distinct saccade appeared in about
90 percent of search trials. Subsequently, scan paths were performed in reading-like direction
from the top left corner row-wise to the bottom right corner. As shown in Figure 2a the relative
frequency of saccades in reading-like direction (.62) was higher than the relative frequency of a
reverse directed saccade (.38). An ANOVA reported a significant difference between these
relative frequencies (F(1,2310) = 4702.65, p < .001). Eye-tracking data, moreover, reported that
fixations on highlighted items were significantly more frequent than fixations on non-highlighted
items. To assess the fixation ratio, we applied a coefficient Φ to obtain a meaningful value of the
portion of fixations on an item type in relation to the total number of fixations and the portion of
this type in the search panel. Φ combines the values of the number of total fixations φT, the
number of fixations on one item type φI, the total number of items in the display ΙT, and the
number of items of a type ΙI as
§ ·
Φ =
¨ ¸
© ¹ .
Φ results in values between 0 and 1, whereat 0 means a low Φ-frequency of fixations and 1 a
high frequency. The coefficient is 0 in two conditions: (a) If no fixations occur on an item type or
and/or (b) if the number of items of a type equals the number of total items.
We applied this coefficient on highlighted words (ΦH) and on standard, non-highlighted
words (ΦS). As shown in Figure 2b, the Φ-frequencies were higher at highlighted words
(Φ = .19) than at non-highlighted words (Φ = .14). An ANOVA reported a significant difference
between the Φ-frequencies of highlighted objects and standard, non-highlighted words
(F(1,2310) = 144.42, p < .001).
(a) (b)
Highlig hted Sta ndard
Re lative fixatio n freque nc y
Read ing-d ire ction Reverse direction
Re lative sac cad e fre qu en cy
Figure 2. Panel (a) shows the relative frequencies of fixations highlighted items in contrast.
to non-highlighted items. Panel (b) shows the relative Φ-frequencies of saccades in reading-
like and reverse directions.
3.4. User Satisfaction
In terms of software usability, user satisfaction is a crucial factor. To investigate not only
performance but also user satisfaction, we analyzed the participant’s rating of three questions at
the end of the test session. As summarized in Table 1, presenting 15 percent of highlighted items
in the search panel resulted in higher user satisfaction throughout. Summarized, the 15-percent
group yielded a rating of 4.00 (SD = 1.35), the 0-percent group a rating of 3.62 (SD = 1.10), and
the 49-percent group a rating of 2.75 (SD = 1.13). The differences between the groups was
significant (X2(10, N = 315) = 70.08, p<.01).
Highlighting Density
0% 15% 49%
In your opinion, how good could you determine whether
the target was present or not? 4.03 4.48 2.63
How convenient was the presentation of the search panels? 3.74 4.00 2.60
How easy was it to determine whether the target was
present or not? 3.09 3.54 3.03
Table 1. This table shows the average rating on a six-step rating scale;
1 corresponds to a negative rating, 6 is the best rating.
4. Discussion
It’s an important aim to increase search speed, accuracy, and user satisfaction of search tasks.
With the current experiment we could demonstrate that this can be achieved by presenting an
optimal density of highlighting. It’s a remarkable finding, since when measuring speed and
accuracy one would expect a speed-accuracy trade-off. As the results show, on average no such
trade-off was found. Longer response times did not result in higher accuracy. The reason for this
effect might by be the limited effort a searcher spends on surveying a display. Such limitations
are described in the Information Foraging Theory [9] which refers to cost-benefit analyses of
information’s value and the expected efforts to reach this information. Consequently, searching is
mostly a hasty and rough scanning of contents accompanied by fast decisions on the relevance of
contents. For the current experiment this means that participants clearly limited their efforts;
although participants were asked to search fast and accurate, none fixated all or at least almost all
word in the search panel. On average 65 fixations were applied to scan through the 140 words
within about 19 seconds. Within these limitations, a speed-accuracy trade-off does not
significantly impact on search performance.
In summary we found clear indications for the hypothesized mechanisms of visual search.
Highlighting facilitates - independent from its validity - the pre-attentive creation of activation
maps. The peaks of activation are fixated serially in an attentive stage of vision. As the results
show, fixations on highlighted items were significantly more frequent than fixations on non-
highlighted items. The deployment of fixations generally occurred in reading-like direction from
the top left row-wise to the bottom right. These results might be valid only for western culture. It
would be an interesting question for future research to compare search paths of different cultures.
Eye movements along the described search paths enable target detection either if the
target itself is highlighted or, by means of peripheral vision, if the target surrounds a fixated item.
Across target-present and -absent tasks in the 15-percent group response times were 1.35 seconds
faster than in the 0-percent group and 2.32 seconds faster than in the 49-percent group. For
target-present tasks, the 15-percent group yielded 2.23 faster response times than the 0-percent
group and 3.07 seconds faster response times than the 49-percent group. Moreover, the 15-
percent group reported 12 percent higher accuracy than the 0-percent group and 14 percent higher
accuracy than the 49-percent group.
The question remains, whether 15 percent highlighting is actually the optimal density for
the current search tasks. This value was arbitrary selected on the basis of a previous study, which
reported a distinct plateau of search performance between 15 and 30 percent highlighting. Future
research should address the question, which density is optimal for different kinds and different
amounts of material.
These results, furthermore, have significant practical impact. Referring to basic
mechanisms of human visual perception and attention, visual search speed as well as search
accuracy can be significantly improved with very simple methods. A crucial factor is that not
only performance can be increased but also a searchers satisfaction with a search episode and the
convenience with the presentation of contents.
This leads back to the sensationalism of this article’s title. It’s an interesting fact, that
Google, likely the most successful and popular Internet search engine, presents the search results
in a way that corresponds quite well to an optimal density of highlighting as reported in the
current and in previous [5] experiments. For example, the percentage of highlighting (bolding,
blue and green colors) of the first 10 results for when searching for “usability” is about 23%. Of
course, it’s far over-the-top to assume that Google’s design of search results, which by now is a
very popular design, is the only reason for the success of the company. But the results of the
current experiment suggest that highlighting contributed to that success to a certain extent.
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ResearchGate has not been able to resolve any citations for this publication.
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