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the conditions for both dependent variables are prob-
ably within tolerable real-world limits for most prac-
tical applications. As discussed above, mean
differences of less than .7 sec are probably meaning-
less in most gross motor tasks. Cell means for both
dependent measures are plotted for this combination
of parameters on Figure 7. The figure indicates stable
performance under these parameters.

CONCLUSIONS

Our results support the use of amplitudes 75% or
greater with frequencies from 2 to 4 Hertz and text
size 0.15 inch or greater for optimum highlighting
value in visual search tasks,

Our conservative recommendations for optimal
blink characteristics (among those tested in this ex-
periment) were based on the following criteria:

1) Maximum accuracy, minimum response times,
and standard deviations of participants observed
under each experimental condition

2) Knowledge of minimum response times with
optimal target highlighting (e.g., under the con-
trol condition)

3) Statistical significance of performance differences

4) Practical implications of observed differences
with regard to real-world applications

5) Preference of erring on the side of being too
conservative and cautious especially in safety-
critical applications

The reader is encouraged to make blink parameter
selection decisions appropriate for particular
applications.

To summarize, research has demonstrated that
blinking is beneficial in reducing search time in
monitoring tasks. Furthermore, blinking has an added
benefit for attracting attention given particular para-
metric values of blink rate, amplitude, duration, and
target size. Knowledge of those parameters may be
useful whether blink is used as a single design feature
or as a redundant cue. For example, when color is
designed to facilitate task performance by attracting
attention to information, and blinking is used as a
redundant cue for color deficients. Blinking can be
used in many forms on modern CRT displays and can
have many real-world applications. Traditionally,
blinking has been used to convey warning — such as
traffic lights, or urgency — such as on fire trucks and

police cars. To some degree, that precept or assump-
tion translates across cultures. So, even though this
study examines blink in a somewhat limited context,
broader application of these findings may be pertinent.

REFERENCES

Anastasi, D.L., Hill, B.P., Murphy, E.D., Cardosi,
K.M., Guttman, J.A., & Amaldi, P. (1995). Com-
puter-Human Interface (CHI) Considerations in
Human Factors in the Design and Evaluation of Air
Traffic Control Systems. DOT/FAA/RD-95/3.
Office of Aviation Research, Washington, DC
20591.

Bailey, R.W. (1989). Human performance engineer-
ing: using human factors/ergonomics to achieve
computer system usability. (2"4 Ed.) New Jersey.
Prentice-Hall.

Boff, K.R. & Lincoln, J.E. (Eds.). (1988). Engineering
Data Compendium: Human Perception and Per-
formance. AAMRL, Wright-Patterson AFB, OH

Christ, R.E. (1975). Review and analysis of color cod-
ing research for visual displays. Human Factors.

17(6) 542-70.

Gerathewohl, S.J. (1951). Conspicuity of flashing and
steady light signals. I. Variation of contrast. USAF
School of Aviation Medicine, Special Report.

Gerathewohl, S.J. (1952). Conspicuity of flashing and
steady light signals. I]. High contrasts. USAF
School of Aviation Medicine, Special Report.

Gerathewohl, S.J. (1953). Conspicuity of flashing and
steady light signals: Variation of contrasts. Jour-
nal of Optical Society of America. 43, 567-71.

Gerathewohl, S.J. (1954). Conspicuity of flashing light
signals of different frequency and duration. Jour-
nal of Experimental Psychology. 48 (4) 247-51.

Gilmore, W.E., Gertman, D.I., and Balckman, H.S.
(1989). The user-computer interface in process con-
trol: A human factors engineering handbook. New
York: Academic Press.

Mertens, H.W. & Milburn, N.J. (1998). Effects of
color vision deficiency on detection of highlighted
targets in a simulated air traffic control display.
Unpublished manuscript.




TABLE OF CONTENTS

PAGE
INTRODUCTION .u..cccccsssesssssesssesesseesseneenesussnessssncseeseesseresssesssnssscseaseseesssecssssssessneseeseeseseeesesaeeeeee 1
METHOD wosessccsessessessesseeressesecseesesssesesssessanesscssssusseseesesssssssscssesussussseaeesasesseauesuesecansaseneeneaneaeateateneeee 2

RESULTS

APPENDIX A: LIST OF FIGURES

Percent correct as a function of frequency, amplitude, text size, and blink duration.............0 Al
Percent correct as a function of frequency and amplitude
Percent correct as a function of frequency, amplitude, and text size
Response time as a function of frequency and amplitude
Response time as a function of frequency, amplitude, and text size...
Response time as a function of frequency, amplitude, text size, and blink duration .
Response time and percent correct as a function of frequency, amplitude, text size,
and blink duration .........ccessssssessesssesseessessesssessuessuessvessesssecsessuesanecsessseeaserssenaesnessueenessneenesaneeseeeses A7

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APPENDIX B: LIST OF TABLES

Age of participants with cell and marginal means.
Design of study
Control condition mean response times for subjects in each of the between-subject groups.....
Descriptive statistics for total percent correct as a function of text size ....

Percent correct as a function of duration under the 2, 3, and 4 hertz and amplitudes of
75 and 100%
Model comparisons and F-statistics for accuracy analysis
Descriptive statistics for total response time as a function of text size

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Table 4: Descriptive Statistics for Total Percent Correct as a Function of Text Size

Small Medium Large
[Ampuirupe | Hz] Min] Mean] SD | MI] Mean] SD | MIN | Mean] SD
25% Amp | 1
2
3
4
50% Amp | 1
2 94. 98.61
3 100.0 | 100.0 0 100.0 | 100.0
4 94.44 | 99.53 | 1.60 | 100.0 | 100.0 0
75% Amp | 1 94.44 | 99.07 | 2.16 | 94.44 | 99.53 1.6
2 100.0 | 100.0 0
3] 100.0 | 100.0 0 94.44 | 99.53 : 1,60
4 | 100.0 100.0 0 100.0 | 100.0 0 94.44 | 99.53 1.60
100% 1 | 94.44 | 99.07 | 2.16 | 94.44 | 99.53 | 1.60 | 100.0 | 100.0 0
Amp 2 | 94.44 | 99.07 | 2.16 | 100.0 | 100.0 0 94.44 | 99.53 1.60
3 | 100.0 | 100.0 0 100.0 | 100.0 0 100.0 | 100.0 0
4 | 100.0 | 100.0 0 100.0 | 100.0 0 100.0 | 100.0 0

* On all conditions at least one person achieved a maximum of 100% correct; therefore, the maximum

range is omitted from this table.

* Cells in which the minimum performance is less than 90% correct have been shaded on Table 4 to

serve as visual aid relevant to the task of establishing a cut-off point for purposes of making

recommendations for blink parameters.
* N= 12 in each text size group

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RESULTS

The SPSS General Linear Model (GLM) repeated
measures procedure was used to analyze the data. All
multivariate tests reported used Pillai’s criterion be-
cause that test pools the statistics from each dimen-
sion to test the effect, and as a result, is more robust
to violations of the assumption of homogeneity of
variance-covariance matrices. The data were ana-
lyzed separately for search errors (percent correct)
and response time, and then summarized considering
the combination of performance measures and blink
parameters relative to optimal performance observed
under the control condition.

The primary purpose of these analyses was not to
detect an effect due to one of the repeated measures
or the between-groups factors; but rather to isolate
the conditions among which there are no differences
in performance. As a result of isolating those condi-
tions, we can identify the combination of blink
parameters that will consistently capture the atten-
tion of the user to promote optimal performance.

Control Condition Analysis

One participant made 1 error in the control con-
dition. Our supposition concerning that error is that
it was an accident or a lapse in attention. The control
condition did not involve searching for targets among
distracters, so it is hypothesized that it was not a
search or identification error. The response times for
the control condition were very short so a number of
repetitions occurred in a small amount of time mak-
ing it likely that the individual did not verify that the
target was selected before terminating the task. As-
suming that it was an accident, this information gives
us some idea of the probability of such accidental
errors happening in the experimental conditions.
That one error occurred from a total of 648 responses
(18 trials times 36 participants), or less than .002
percent of the time. The mean percent correct for the
36 participants was 99.802 with a standard deviation
of 1.19. No significant response time differences
between the six groups in the control condition were
found using an ANOVA. Table 3 includes the mean
response time for each group under the control con-
dition, which ranged from 3.98 sec to 5.23 sec. with
an overall average of 4.75 sec.

Analysis of Search Errors

Frequency and amplitude. Both frequency [F, ,,
=7.65,a = .05, p= .001] and amplitude [F, ,, =8.16,a
=. 05, p< .001] were significant as main effects in
error data analysis, and the interaction between fre-
quency and amplitude was significant [F, ,, =3.04,a=
.05, p= .016]. Figure 1 illustrates the lower percent
correct responses under the 1 Hertz condition and
the gradually improving percent correct for 2, 3, and
4 Hertz conditions. The 25% amplitude condition
showed the most improvement (increase of percent
correct) with the faster frequencies. However, the
poor attention-getting attributes of the 25% and
50% amplitudes are evident across frequencies when
compared to the larger amplitude conditions.

Size and Duration. Table 4 presents the percent
correct averaged over the 2 blink durations because
duration was not found to be a significant factor in
the analysis involving the full model (4 levels of
frequency, 4 levels of amplitude, 3 sizes, and 2
durations). The percent correct ranged from 22.22%
for the smaller amplitudes and slower frequencies
with small and medium text to an average of 100%
correct in the most attention-getting conditions. In
all frequency conditions, the 25% amplitude condi-
tions had a lower percent correct compared to the
larger amplitudes for that frequency. It is important
to note that in each experimental condition at least 1
person was able to achieve a 100% correct score;
therefore, inclusion of the maximum range column
on Table 4 was unnecessary.

Table 4 was included primarily to provide infor-
mation relevant to the distribution of accuracy scores
under each condition. For example, in the most
attention-getting conditions, the standard deviation
for participants ranged between 0 and 2.16. How-
ever, the standard deviation among participants was
as high as 30.0 for the 2 HZ, 25% amplitude condi-
tion with small text. The minimum scores have been
included for each condition to demonstrate the range
of performance and to facilitate selection of blink
parameters that will accommodate most users. Cells
in which the minimum performance is less than 90%
correct have been shaded on Table 4 to serve as a
visual aid relevant to the task of establishing a cut-off
point for purposes of making recommendations for
blink parameters. Previous research (Milburn &


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OPTIMIZING BLINK PARAMETERS FOR HIGHLIGHTING
AN AIR TRAFFIC CONTROL SITUATION DISPLAY

INTRODUCTION

A primary consideration of any use of computers
is the presentation of information flowing from the
computer to the human in a concise form that is
quickly and easily interpreted. The varied research
that is inspired by that topic sentence is extensive,
including screen design, computer messages, com-
puter response time, types of output, types of dis-
plays, resolution, refresh rates, luminance and contrast
just to name a few. Specifically, this study examined
the effectiveness of blinking or flashing of text as a
method of gaining the user’s attention.

Though researchers and display designers cannot
agree on a term for the design feature (e.g. blinking,
flashing, flickering, winking, pulsing, and off-on
cycling), several sources (Anastasi, Hill, Murphy,
Cardosi, Guttman, & Amaldi, 1995; Boff & Lincoln,
1988; Christ, 1975; Gerathewohl, 1951, 1952, 1953,
1954; Gilmore, Gertman, & Blackman, 1989; Mili-
tary Standards, 1989; Thackray & Touchtone, 1991;
Van Orden & Di Vita, 1993) agree that blinking
targets are more alerting than steady-state targets and
can aid the user in finding the targets quickly. Those
sources have recommended information coding di-
mensions such as target size, color, shape, brightness
contrast, and frequency, but no guidelines have been
recommended fora minimum blink amplitude that is
most effective for attention-getting. Because those
studies involved blinking characteristics in which the
targets alternated between on and off (100% change
in intensity) a recommendation of 100% amplitude
change during blinking was implicit.

A previous study (Mertens & Milburn, 1998)
evaluated the effectiveness of redundant color coding
for protecting the performance of individuals with
color vision deficiency in a simulated air traffic
control search task requiring search and identifica-
tion of specific aircraft. Blinking was used along with
color-coding as one of the redundant cues. The
blinking characteristics of 20% amplitude modula-
tion, 2 hertz (HZ) blink rate, and .1 sec blink dura-
tion were used in that study to simulate those used in

the developmental Initial Sector Suite System (ISSS),
a system now discontinued, that was to provide a new
work station for controllers. Other coding dimen-
sions and conditions of that study also simulated
conditions of the ISSS. We do not know why those
characteristics of blinking were selected for the pro-
totype displays. (Those characteristics of blinking
may have been selected to reduce the potential dis-
traction effects of blinking, or to reduce a possible
adverse effect of blinking on legibility.) The results of
our study demonstrated that the 20% decrease in
brightness intensity at a 2 HZ rate was of little or no
attentional value for both individuals with normal
and abnormal color vision (Mertens & Milburn,
1998).

As mentioned above, our initial interest in blink-
ing was based on examination of early prototypes of
new air traffic control displays and the use of blinking
as a redundant cue used with color. Our primary
research concern deals with development of guide-
lines for use of color on CRT displays that would
allow color deficient users to adequately use the
color-coded information. Guidelines for design of
electronic displays recommend that color coding
should always be used as a redundant cue, such that
some other characteristic of the symbol, shape, size,
brightness, alphanumerics, blinking, and so forth,
conveys the same information (Silverstein &
Merrifield, 1985). One of the stated benefits of
redundant color-coding is that it “...permits people
with color vision deficiencies (CVD) to interpret
color coded displays” (Society of Automotive Engi-
neers, 1988). Redundant color-coding can have func-
tions other than coding, such as decluttering
information and attention-getting. When the use of
color is for the latter purposes, the attention-getting
value of the redundant cue or cues must be consid-
ered. Therefore, we conducted a study that focused
on the attention-getting value of blinking amplitudes
to determine how much ofa change in brightness was
necessary to reliably capture the user’s attention if no
other redundant cues were available. Our intent was




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of known response time differences related to age,
participants were matched by age for the six groups.
An ANOVA was conducted to determine the effec-
tiveness of the placement of participants to design
cells with regard to age. No significant differences
were found. Table 1 presents participant ages and
mean age for each group. Participants placed in the
small and medium text groups had a mean age of 23.3
years and the large text group had a mean age of 25.5
years. The people who served as subjects in the .1 sec
duration group had a mean age of 23.9 years and
those in the half-cycle group averaged 24.2 years old.

All volunteers had at least 20/30 corrected visual
acuity in both near and distant vision as determined
with the Bausch and Lomb Orthorater. Normal color
vision was determined by the Ishihara’s Test for
Color-Blindness 24-Plates Edition, and Farnsworth’s
F-2 PIC Plate test.

Design

The mixed model experimental design included 4
blink frequencies (1, 2, 3, & 4 Hertz), 4 Blink
amplitudes (25%, 50%, 75%, & 100% reduction in
brightness), 2 blink durations (off or dim times of 0.1
sec and one-half of the blink cycle), and 3 sizes of text
(character heights of 0.10, 0.15, & 0.20 inch). Am-
plitude and frequency were repeated measures, and
duration and size were between-subjects factors. Table
2 provides a graphical representation of the design
and number of participants within each cell.

Procedure

The ATC situation display was presented on a
Tektronics 4125 19-inch monitor and consisted pri-
marily of aircraft symbols, each with a block of
alphanumeric data attached by a leader line to the
aircraft symbol. The alphanumeric data, called the
data block, contained information about the state of
the aircraft, e.g., call sign, aircraft type, heading,
altitude, and ground speed. The aircraft symbols and
their data blocks were positioned throughout the
situation display. Map data that designated bound-
aries of air traffic control sectors, geographic data,
and navigation references were also included in the
situation display. Participants were instructed to lo-
cate all of the blinking data block targets, to select
them using a mouse, and to press the “enter” key to
advance to the next trial. They searched for 1 or 2
targets, within a field of 16 aircraft (data blocks), on
each of the simulated ATC situation display. Location

of targets was randomized in each of 8 areas or
divisions of the screen with each combination of
areas appearing once in 2 target trials. Data blocks
other than the blinking targets were called distracters.

Each experimental condition consisted of 1 dem-
onstration trial to illustrate the amplitude of the
blink for the targets in that condition, followed by 5
practice trials and 18 test trials. Fourteen trials con-
tained 2 targets, and 4 trials contained 1 target.
Participants were informed that some trials would
present screens with only 1 target. Targets were not
redundantly coded, so identification of targets was
based on the participant’s perception of the blinking
parameters.

A control condition was presented to obtain a
baseline and an estimation of the optimum response
time. The control condition consisted of trials with
display screens that were identical to those presented
in the experimental conditions, except that they
contained no distracter targets. Each screen con-
tained either 1 or 2 data blocks. All data blocks on the
screen were steady, but were considered the targets,
and the participant’s task was to select them as quickly
as possible. The targets were displayed on the same
background map as used for trials with distracters,
and the locations of targets were varied by the same
techniques as for trials with distracters.

The 4 amplitudes of blink were 100%, 75%, 50%,
and 25%. For this study, target blink amplitude is
defined as the percentage of decrease in luminance
from a standard luminance of 51.4 Cd/m?. Frequen-
cies of 1, 2, 3, and 4 Hertz (HZ) were used. The
duration of the blink intensity decrement was either
1/10 sec or half of the blink cycle time (half-cycle).

The instructions urged the participants to respond
quickly and accurately, with the emphasis on accu-
racy. All conditions were self-paced and four 5-
minute breaks were programmed into each
participant’s unique random order of conditions, so
that the breaks were evenly spaced throughout the
experiment. All participants completed the experi-
ment within 4 hours.

Performance Measures

Performance was measured in terms of accuracy
(percent correct) and search time. Search errors included
misses (failure to select a target), and false alarms (selec-
tion of non-targets). The response time measure was
defined as the elapsed time between the onset of the trial
and the subject’s key-press-termination of the trial.


APPENDIX B

Table 1: Age of Participants with Cell and Marginal
Means

Half-Cycle .1Sec Marginal Size
Means

Smaill .10 inch

18 19
20 19
20 21
25 23
29 25
31 30
X= 23.83 X = 22.83 X= 23.33

Medium .15 inch

19 18
19 19
21 19
21 25
27 28
34 30
X= 23.50 X= 23.17 X= 23.33

Large .20 inch

18 18
21 22
22 23
31 23
34 34
34 34
X= 25.16 X= 25.83 X= 25.50
Half-Cycle -1sec Grand
mean mean Mean
X= 24.17 X= 23.94 X = 24.0556

BL


DOT/FAA/AM-99/8

Office of Aviation Medicine
Washington, D.C. 20591

Optimizing Blink Parameters
For Highlighting an Air Traffic
Control Situation Display

Nelda J. Milburn
Henry W. Mertens

Civil Aeromedical Institute
Federal Aviation Administration
Oklahoma City, Oklahoma 73125

March 1999

Final Report

This document is available to the public
through the National Technical Information
Service, Springfield, Virginia 22161.

@

U.S. Department
of Transportation

Federal Aviation
Administration


Finally, in an analysis involving 2, 3, and 4 HZ, 3
text sizes and the 75% and 100% amplitude condi-
tions; amplitude, frequency, and text size were no
longer significant as main effects or as interaction
effects. The results of Step 4 suggest that the larger
amplitudes of a blinking target have potent, atten-
tion-getting value in a search task. However, an
accuracy performance difference related to the dura-
tion of the blink emerged. Table 5 shows no errors
were made under the .1 sec duration condition at 2,
3, and 4 HZ with 75% and 100% amplitudes, and a
total of 10 errors was made by the 18 people who
served as participants under the half-cycle blink du-
ration condition. Most errors occurred under the 2
and 3 HZ conditions; only 1 participant made | error
on the 18 trials presented under the 4 HZ frequency.

A summary (Table 6) of these model comparison
results is provided to aid the reader in understanding
the blink parameters that produced essentially the
same accuracy performance and those that did not.

Analysis of Search Response Times

Initially, the full model was analyzed, and the
following 4 sections briefly discuss the findings rel-
evant to main effects and interaction effects of fre-
quency, size, duration and amplitude. Following
those individual topics, a discussion of the model
comparisons procedure similar to the one conducted
on the accuracy data is addressed for the search
response time analysis.

Frequency. Frequency of blink was significant [F,
9g 749.23,a=. 05, p< 001] and is especially noticeable
in the pattern of response times for the 25% ampli-
tude across different frequency conditions (Figure
4). As the frequency increased, the response times
gradually decreased for all amplitudes, with the 25%
amplitude conditions showing the most reduction in
response time at the 4 HZ frequency.

Size. Size of text was significant as a between-
groups factor [F, ,, =3.66,a= .05, p= .038] and there
were significant interactions of size with frequency
[F, 5, =3-51,a= .05, p= .005], and size with amplitude
[F,, 5 =5-02,a= .05, p< .001]. These interactions can
be seen in Figure 5. The slower response times at the
25% amplitude, compared to the 75% and 100%
amplitudes, combined with the distinct pattern (slow
to faster) response times for small, medium, and large
text across frequencies is evident in Figure 5. The
consistent pattern of responses found in this study
supports the theory that size of target text is important

if the blink frequency is infrequent (does not occur
during a typical dwell time during a visual search).
Furthermore, if the amount of change in brightness
is subtle, as noted by the longer search times in the
1HZ-25% amplitude condition, the large text has a
definite advantage over the smaller sizes.

Duration. In the analysis involving the full-facto-
rial model (4 frequencies, 4 amplitudes, 2 durations,
and 3 text sizes), duration was not significant. See
Figure 6.

Amplitude. As a main effect, amplitude was sig-
nificant [F, ., =22.79,a= .05, p< .001] with the
greatest mean response-time differences under the
25% amplitude condition. Figures 4 and 5 portray
the disparity between the amplitude conditions. There
are very small mean response time differences be-
tween the 75% and 100% (.05 sec) amplitude condi-
tions, with larger differences between the 50% and
75% amplitudes (.79 sec), and the greatest difference
is between 25% and 50% amplitudes (4.68 sec). It is
clearly evident that for most applications, a .05 sec
response time difference (between 75% and 100%
amplitudes) has little or no significance from a prac-
tical application viewpoint. Furthermore, using that
same criterion to determine practical significance, a
4.68 sec difference between 25% and 50% ampli-
tudes may be consequential in time-critical, safety
applications. However, when that same question is
applied to the 50% and 75% interval, the interpreta-
tion of significance is more difficult. Statistically, the
-79 sec difference is significant, IF, 59 = 20-92,a= .05,
p< .001] but depending upon the application to
which these results are generalized, acceptability will
obviously depend upon the criticality of that length
of mean response time and the response accuracy
associated with the amplitude condition.

Descriptive Statistics for Response Time.

Table 7 presents the descriptive statistics (mini-
mum, maximum, mean and the standard deviation)
for response time under the experimental and control
conditions. There are many ways of evaluating the
response time performance of the experimental con-
ditions. Bailey (1989) recommends designs that will
accommodate 95% of the users by considering the
full range of measurements. If that advice is applied
to the response time data and the control condition
is used as a baseline of response time, the 95" percen-
tile response time is 8.16 sec. It seems reasonable that
the mean for the experimental conditions should be


Mertens, 1997; Mertens & Milburn, 1998) indicated
that a blink amplitude of 25% is not effective as an
attention-getting mechanism; and, conversely, ample
research (Anastasi, Hill, Murphy, Cardosi, Guttman,
& Amaldi, 1995; Boff & Lincoln, 1988; Christ,
1975; Gerathewohl, 1951, 1952, 1953, 1954;
Gilmore, Gertman, & Blackman, 1989; Military
Standards, 1989; Thackray & Touchtone, 1991; Van
Orden & Di Vita, 1993) has demonstrated that an
on/off blink (100% amplitude) is effective. In a study
(Milburn & Mertens, 1997) with blink parameters of
2 HZ, .1 sec blink duration, and 7 blink amplitudes
ranging from 12.5% to 100%, performance was
stable for the 75% amplitude condition and greater,
and was clearly unacceptable for conditions of blink
amplitude less than 50%. In that study, a paired-
sample t-test was used to determine whether a signifi-
cant difference existed between the 50% and 75%
amplitude conditions. No significant difference was
found, but a conservative recommendation to avoid
amplitudes less than 75% was made based on the
small sample size of that study and the marginally
acceptable performance obtained for the 50% ampli-
tude condition. Therefore, based on results of that
study and because clear-cut dividing lines are not
readily apparent in this study’s data, further analyses
were conducted to provide more information perti-
nent to making blink parameter recommendations.
Model Comparisons. The omnibus test described
above was conducted to determine if performance
differed as a result of varying frequency, amplitude,
duration and text size, and many differences were
found. The omnibus test answers the question, “Are
any (performance) differences present among the
levels of the independent variables?” However, it
cannot identify which levels are significantly differ-
ent from another level, and under which levels the
performance is the same based on the resulting F-
statistic. Therefore, in an effort to isolate the condi-
tions with essentially the same performance (near
optimal) from sub-optimal conditions, a test of fo-
cused contrast was used. That technique involves
analyzing a sub-set of a complex design by systemati-
cally omitting levels of a factor from the analysis.
This is particularly useful when the number of levels
is large or the design is complex. The levels represent
arange of conditions (1 to 4 Hertz, and 25% to 100%
amplitude) so the strategy was to eliminate the

minimum level first, analyze the remaining factors to
test for a difference, and if found, then proceed by
dropping the next higher level. The final step was to
test adjacent levels of a factor to find break points at
which performance differs. At least with this experi-
ment, it would be advantageous from a CRT display
design standpoint ifa “cut-point” existed above which
performance was stable and near optimal and below
which performance was clearly unacceptable. Realis-
tically, that cut-point is usually not well defined,
especially in complex designs and if more than one
dependent measure is involved. The description of
the analyses that follow was an attempt to discover
that cut-point for each of the two dependent variables.

As mentioned above, the first step was to conduct
an omnibus test and to examine the results. The
descriptive statistics of this study were consistent
with the results from the pilot research. Evidence
from those two studies indicates that the 25% ampli-
tude condition produced unacceptable performance
relative to the other amplitudes. See Figure 2. Con-
sequently, that level of amplitude was dropped from
further analysis. Subsequent analyses of the response
accuracy measure focused on evaluating the 3 re-
maining levels of amplitude and 4 levels of frequency.

The second step of the model-comparison proce-
dure involved an analysis of the 4 frequencies, 3 text
sizes and the 50% and 75% amplitude conditions, to
answer the hypothesis question related to the effec-
tiveness of the 50% amplitude condition. Signifi-
cance was found for frequency [F, ,, =5.05,a= .05, p=
-006], amplitude [F,, jo =4-1,a= .05, p= .052], fre-
quency by amplitude [F, ,, =4.64,a=. 05, p= .009],
anda between-groups effect due to size [F, ,, =4.89,a=
05, p= .015]. Figure 2 illustrates the interaction
between frequency and amplitude; likewise, Figure 3
graphs the percent correct measure as a function of
frequency, amplitude, and text size.

Step 3 was conducted to determine ifa significant
difference existed between the 50% and 75% ampli-
tude conditions if the 1 HZ condition were dropped.
Therefore, another model comparison procedure was
run that involved only those 2 amplitudes and 3
frequencies (2, 3, and 4 HZ) and 3 text sizes. The
results revealed a significant interaction between fre-
quency and amplitude [F, 9) =4.2,a= .05, p= .025], no
significant between-groups factors, and no significant
post-hoc multiple comparisons for the size factor.


to use the resulting information in the preparation of
another study examining the use of blinking as a
redundant cue with color.

Our first study on blink amplitudes (Milburn &
Mertens, 1997) used an air traffic control display
similar to the one used in this experiment except that
only the amplitude variable was manipulated. For
that study, target blink amplitude was defined as the
percentage of decrease in luminance from a standard
of 51.4 Cd/m? ata rate of 2 HZ, and the duration of the
blink intensity decrement was approximately 1/10 sec.
The 7 amplitudes of blink were 100%, 93.75%,
87.5%, 75%, 50%, 25%, and 12.5%. Some blink
amplitudes had such poor attention-getting value
that 2 respondents were unable to find even a single
target of the 26 possible during an 18-trial experi-
mental condition. On the average, participants made
very few false alarms for any of the different blink
amplitude levels. Most errors were misses, defined as,
a failure to detect and select a blinking target. When
the targets were difficult to detect, some respondents
terminated the trial without finding any targets, or
without finding all of the targets on the trials that
contained multiple targets. High miss rates and low
false alarm rates were typical in conditions involving
a small change in amplitude ranging from 12.5% to
25%. Of the types of errors discussed, misses are the
most critical in air traffic control work. A false alarm
might result in the air traffic controller making some
additional mental calculations, with a loss of time, but
failure to quickly find an emergency warning or critical
event displayed on the ATC situation display could
result in a loss of required separation between aircraft.

Performance in that study was stable with high
accuracy ranging from 98.75% to 100% correct for
conditions involving a 75% to 100% decrease in
brightness but showed a steady increase in errors
(missed targets and selection of non-targets) for the
50%, 25%, and 12.5% decreases in brightness. Per-
formance in conditions involving amplitudes of 25%
or less ranged from 5% correct to 72.5% correct.

The objective of Milburn and Mertens’ (1997)
study was to determine the level of blink amplitude
that can capture the user’s attention quickly and with
a high degree of regularity (at least 95% of the time).
Note that others have arbitrarily used 95% correct
criterion for selection of cue parameters in the design
of symbology for aviation systems (Silverstein &
Merrifield, 1985). Milburn and Mertens (1997) found

a strong relationship between accuracy and response

time under each of the blink amplitude conditions.
Unwavering accuracy and response time performance
were maintained for amplitudes ranging from 75% to
100%, and a steep decline in accuracy, combined
with increased response times, was evident for the
amplitudes ranging from 12.5% to 25%.

Results from Milburn and Mertens (1997) aided
the design of this study and our attempt to address
the complex issues surrounding the selection of opti-
mal attention-getting blink parameters. The studies
are alike in that similar methodologies were used;
however, this experiment addressed a more complex
set of parameters relevant to designing ATC situ-
ational displays. The first experiment examined a
range of blink amplitudes in order to determine
whether performance differences exist between blink
amplitudes. We selected 4 blink amplitudes (25%,
50%, 75% & 100%) that were used in the first study
to use in a more complex design intended to examine
the interaction of blink amplitude with other blink
characteristics such as frequency, duration and text
size. This study expanded the design of the first by
adding frequency as a factor (1, 2, 3, & 4 Hertz),
comparing 2 blink durations (off or dim times of 0.1 sec
and one-half of the blink cycle), and comparing 3 sizes
of text (character heights of 0.10, 0.15, 8 0.20 inch).

Concisely stated, the purpose of this study was to
examine the interaction of blink amplitude with
frequency, duration and size of text and to select
optimal attention-getting blink parameters from
among those tested.

METHOD

Participants

Prior approval for all procedures and use of human
participants was obtained from the Office of Avia-
tion Medicine/Civil Aeromedical Institute human
use committee. Volunteers were recruited by the
Human Resources Research Division of the Civil
Aeromedical Institute. The informed consent of ev-
ery participant was obtained prior to participation,
and each participant was free to withdraw from the
experiment without prejudice at any time during the
experiment.

Thirty-six participants ranging in age from 18 to
34 years of age served as participants in this study.
Participants were matched by gender and age for each
of the between-subject groups (text size and dura-
tion). Eighteen of the participants were male. Because


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Table 5: Percent Correct as a Function of Duration Under the 2, 3, and 4 Hertz
and Amplitudes of 75 and 100%

FREQUENCY/AMPLITUDE
DURATION
2 HERTZ 3 HERTZ 4 HERTZ
75% 100% 75% 100% 75% 100%

-1 SEC 100 100 100 100 100 100

HALF-CYCLE 95 83 72 100 94.5 100
* [F,, 30=4.31, p=.047]
Table 6: Model Comparisons and F-Statistics for Accuracy Analysis
Step 1 Step 2 Step 3 Step 4
FREQ AMP SIZE DUR FREQ AMP SIZE DUR FREQ AMP SIZE DUR FREQ AMP SIZE DUR

25%

MULTIVARIATE TESTS

text size,
50% Amp, | Hz small vs. large
text size

text size

MULTIVARIATE TESTS | MULTIVARIATE TESTS | MULTIVARIATE TESTS
Hz [F3,9¢=7.65, p = .001) Hz [F 3,25= 5.05, p = 006] Hz*Amp [F 9.29 = 4.20, p = .025] None
Amp [F 3, 2¢= 8.16, p <.001] 10,
Hz*Amp [F 9, 22 = 3.04, p=
016)
BETWEEN BETWEEN BETWEEN BETWEEN
Size [F 2, 39= 3.50, p = .043) Size [F2,39= 4.89, p = .015] None Duration [F ;,39= 4.31, p = .047]
MULTIPLE MULTIPLE MULTIPLE MULTIPLE
COMPARISONS COMPARISONS COMPARISONS COMPARISONS
25% Amp, | Hz small vs. large | 50% Amp, 1Hz small vs. large None None

B4




Milburn, N.J. & Mertens, H.W. (1997). Evaluation of

a range of target blink amplitudes for attention-
getting value in a simulated air traffic control
display. Federal Aviation Administration (FAA)
Office of Aviation Medicine. Report No. DOT/
FAA/AM-97/10. Washington, DC. U.S. Depart-
ment of Transportation. NTIS #ADA326465.

Military Standards, MIL-STD-1472D, (1989). Hu-

man Engineering Design Criteria for Military
Systems, Equipment and Facilities.

Society of Automotive Engineers, SAE G-10 Commit-

tee, Silverstein, L.D., Jacobesen, A., Post, D.,
Druk, R.V., Mertens, H.W., & Gershzohn, G.
(1988). Human engineering considerations in
the application of color to electronic aircraft dis-
plays. SAE Technical Report ARP4032. Warren-
dale, PA: Society of Automotive Engineers, Inc.

Silverstein, L.D. & Merrifield, R.M. (1985). The de-

velopment and evaluation of color systems for
airborne applications. Phase 1: Fundamental vi-
sual, perceptual, and display system consider-
ations. Technical Report DOT/FAA/PM-85-
19. Washington, DC: U.S. Department of
Transportation.

Thackray, R.I. & Touchstone, R.M. (1991). Effects of

monitoring under high and low taskload on de-
tection of flashing and coloured radar targets.
Special Issue: Cognitive ergonomics II. Ergonomics.

34(8) 1065-81.

Van Orden, K.F. & Di Vita, J. (1993). Highlighting

with flicker: Some basic visual search consider-
ations. Proceedings of the Human Factors and
Ergonomics Society 37th Annual Meeting—1993.
1300-4.


APPENDIX A

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Technical Report Documentation Page

1. Report No. 2, Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-99/8

4, Title ond Subtitle 5. Report Date

Optimizing Blink Parameters for Highlighting an Air Traffic Control March 1999

Situation Display 6. Performing Organization Code

7, Author(s) 8. Performing Organization Report No.

Milburn, NJ., and Mertens, H.W.

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

FAA Civil Aeromedical Institute
P.O. Box 25082
Oklahoma City, OK 73125 11. Contract or Grant No,

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered
Office of Aviation Medicine

Federal Aviation Administration
800 Independence Avenue, S.W.
Washington, D.C. 20591 14. Sponsoring Agency Code

15. Supplemental Notes

This work was performed under task HRR-190.

16. Abstract

Research suggests blinking targets are more alerting than steady targets; however, several factors can interact with
blinking to either improve or degrade its attention-getting value. Those factors include target size, color, brightness
contrast, frequency of blink, and the time the blink is at_ maximum brightness relative to the time it is off or dim
during the blink cycle. No guidelines were found for determining the optimal attention- getting blink amplitude
(the percentage of decrease in target brightness from a standard) and the interaction of blink amplitude with the
other blink characteristics mentioned above. Thirty-six participants were asked to locate and select blinking blocks
of text on a simulated air traffic control display to examine the interaction of blink frequencies, amplitudes, and
durations with size of text. Our results support the use of amplitudes 75% or greater combined with frequencies
from 2 to 4 Hertz and text size 0.15 inch or greater for optimum highlighting value in visual search tasks.

17. Key Words 18. Distibution Statement

Attention-Getting, Text Size, Target, Flashing CRT Document is available to the public through the
Display, Frequency, Air Traffic Control, Amplitude, Blink | National Technical Information Service,
Springfield, Virginia 22161

19. Security Classif. (of this report) 20, Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 26

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized


Table 2: Design of Study

Di
URATION .1 sec (n= 18)
FREQUENCY 1 HERTZ 2 HERTZ 3 HERTZ

4 HERTZ

AMPLITUDE
(IN PERCENT) 25 50 75 100 | 25 50 | 75 100 | 25 50 75 100 | 25 50 75 100

TEXT SIZE

SMALL
n=6
MEDIUM
n=6
LARGE
n=6

DURATION Half-Cycle (n= 18)

3 HERTZ 4 HERTZ

FREQUENCY 1 HERTZ 2 HERTZ

AMPLITUDE

(IN PERCENT) 25 50 75 100 | 25 50 | 75 100 | 25 50 75 100 | 25 50 75 100

SMALL
n=6
MEDIUM
n=6
LARGE
n=6

TEXT SIZE

Table 3: Control Condition Mean Response Times for Subjects in Each of the
Between-Subject Groups

Size Duration
.1 SEC HALF-CYCLE MARGINAL MEANS
SMALL — .10" 4.96 5.23 5.10
MEDIUM .15" 5.10 4.72 4.91
LARGE .20" 3.98 4.50 4.24
MARGINAL,
MEANS 4.68 4.82 4.75

*This table is provided to show equivalent performance for subjects in each of the 6
groups for between-subjects analysis in the control condition.

B2


NOTICE

This document is disseminated under the sponsorship of
the U.S. Department of Transportation in the interest of
information exchange. The United States Government
assumes no liability for the contents or use thereof.


Table 7: Descriptive Statistics for Total Response Time as a Function of Text Size

Small Medium Large

Amp. | Hz | MAx | MIN | MEAN | sp MAX | MIN | MEAN | sp | MAX | MIN | MEAN | sp

Con- | NA | 8.42 | 3.18 | 5.10 | 1.76 | 6.29 | 3.79 | 491 | .78 | 7.36 | 3.22 | 4.24 | 1.16
trol

25% 1
Amp
2
3
4
50% 1
Amp

2 10.87 | 4.30 | 7.71 2.11 8.56 | 4.59 | 6.48 1.44 | 16.82 | 4.02 | 6.06 3.56

3 10.75 | 4.11 7.62 | 2.03 9.07 | 4.79 | 6.43 1.42 | 7.86 | 3.76 | 5.06 1.27

4 | 15.14} 4.92 | 7.84 | 3.01 | 7.95 | 4.37 | 5.89 | 1.04 | 11.08] 3.65 | 5.40 | 2.19

75% 1 12.08 | 4.67 | 6.98 | 1.94 | 8.03 | 4.95 | 659 | 1.01 | 6.93 | 4.26 | 5.53 96
Amp

2 | 955 | 4.32 | 6.06 | 1.61 | 7.67 | 4.53 | 6.15 | 1.21 | 12.41 | 3.76 | 5.64 | 2.34

3 | 15.39] 4.10 | 6.71 | 289 | 9.34 | 4.77 | 6.33 | 1.54 | 14.91 | 3.59 | 5.62 | 3.07

4 10.20 | 4.02 | 5.69 1.67 7.76 | 4.21 5.35 1.03 | 10.35 | 3.42 | 5.12 2.00

100% 1 16.19 | 4.94 | 7.52 | 3.21 | 11.67] 5.30 | 6.80 1.76 | 17.53 | 4.38 6.55 3.63
Amp

2 9.38 | 4.30 | 5.94 1.68 | 8.10 | 4.61 5.66 98 7.00 | 3.81 5.10 1.07

3 | 10.74 | 3.98 | 5.77 | 1.98 | 8.08 | 3.69 | 5.83 | 1.41 | 15.07] 3.68 | 6.02 | 3.44

4 8.22 | 4.26 | 5.62 | 1.32 | 6.64 | 4.05 | 5.32 | 89 | 8.84 | 3.66 | 5.03 ] 1.51

* Cells in which the mean response time is greater than the 95"" percentile (8.16 sec) for the control
condition have been shaded on Table 7 to provide a visual aid relevant to the task of establishing a cut-off
point for purposes of making recommendations for blink parameters.

* N= 12 in each size group

BS


as good as the 95" percentile response time for the
control condition. Therefore, the cells with means
greater than 8.16 sec have been shaded on Table 7.
Granted, this may be a very liberal initial criterion
and may not represent ideal performance, but it will
serve as a starting point for eliminating conditions
with the longest response times. Also, many of the
same experimental conditions are shaded on both
Tables 4 and 7.

Model Comparisons. A 4-step model comparison
procedure similar to the one described for the accu-
racy data was used on the response time data. Once
again, the purpose of these analyses was to uncover a
“cut-off point” at which the performance is statisti-
cally significantly different from the remaining levels
of a factor, and to identify levels of a factor that
exhibit essentially identical and optimal performance.
The technique used is analogous to the 4 steps per-
formed on the search accuracy data. The full model
analysis results described above constituted Step 1.

The analysis of Step 2 involved 4 frequency condi-
tions, the 50% and 75% amplitude conditions and
size as a between-groups variable. Significant effects
were found for frequency [F, ,, = 10.9,a= .05, p<
-001}, amplitude [F, ,, = 29.09,a- .05, p< .001],
amplitude by size [F, 59 = 12.84,a=.05, p< .001], and
frequency by amplitude [F, ,, = 3.45,a=.05, p= .03].

Response times for the 4 frequency conditions and
the 50% and 75% amplitude conditions averaged
over duration were compared using a Games and
Howell multiple comparison procedure to control
alpha for 3 comparisons (at each frequency) to deter-
mine if significant differences exist between the 3 text
sizes. Response time differences were significant com-
paring the large with the small and medium text for
the 50% amplitude conditions both at the 1 HZ and
3 HZ frequencies and the large with medium text for
1 HZ and 75% amplitude.

Step 3 of the analysis involved dropping the 1 HZ
level of the frequency factor and making the first
comparison of adjacent levels of the amplitude fac-
tor. The size and duration factors remained in the
model. Significant factors, interactions and post-hoc
comparisons were undifferentiated from the signifi-
cant results of Step 2 with the following exceptions.
First, because the 1HZ level was deleted from this
analysis, the post-hoc multiple comparisons do not
appear in the list of significant comparisons. Second,
the interaction of frequency by amplitude was not
present in Step 3 of the analysis. However, frequency

[F, ) = 5.75,a= .05, p= 008), amplitude [F, ,. =
20.92,a= .05, p< .001], and an interaction of ampli-
tude with size [F,, 9 = 11.06,a= .05, p< .001] re-
mained on the significance list.

Finally in Step 4, response times for the 2, 3, and
4 HZ, 75% and 100% amplitudes, 2 durations, and
3 text sizes were analyzed. The only significant effect
was due to frequency [F, ,, = 12.36,a= .05, p< .001].
That significance is probably a result of the slightly
faster response times under the 4 HZ conditions. The
mean response times for 2, 3, and 4 HZ are 5.76,
6.05, and 5.36 sec respectively. Once again, small
differences in mean response time (.4 and .69 sec) for
the 4 HZ condition as compared to the 2 and 3 HZ
are statistically significant; but practical implications
should be considered. Furthermore, there were no
significant Games and Howell post-hoc comparisons
for the text size variable.

Table 8 provides a concise summary of the signifi-
cant results found in the 4-step model-comparisons
procedure for the response time dependent measure.

Summary of Results

Blinking has high attention-getting value at 2, 3,
and 4 Hertz when the decrease in brightness is 75%
or greater; accuracy and speed of search were reduced
at lower amplitudes. The recommended duration of
a blink (off or dim time) is typically half of the cycle
time; however, our research indicates that for fre-
quencies of 2 to 4 Hertz, a shorter 0.1 sec blink was
as effective as blink durations that are half of the blink
period. Size of text was statistically significant as a
between-group factor. That significance can be at-
tributed to lower performance under the slower fre-
quencies (1 & 2 Hertz) and the smaller amplitudes
(25% & 50%) for the small to large text size compari-
sons. Blink amplitudes need to be 75% amplitude or
greater to be maximally attention-getting if the text is
smal] (near .10 inch). Blink amplitudes of 50% or
more were sufficient with large text (near .20 inch).

DISCUSSION

The 75% and 100% amplitude conditions consis-
tently produced the lowest errors regardless of the
blink frequencies and text sizes used in this experi-
ment. Furthermore, in Step 4 of the model compari-
son procedure, involving the 2, 3, and 4 HZ
frequencies, 3 text sizes, and only the 75% and 100%
amplitudes; the small performance differences among