DOT/FAA/AM-96/22

Office of Aviation Medicine

Washington, D.C. 20591

A Further\klidation ofthe

Practical ColorVisionTest

for En Route AirTraffic

Control Applicants

H.W. Mertens

N.J. Milburn

W.E. Collins

Civil Aeromedical Institute

Federal Aviation Administration

Oklahoma City, Oklahoma 73125

August 1996

Final Report

Di

This document is available to the public

through the National Technical Information

Service, Springfield, Virginia 22161.

Ö

U.S. Department

ofTransportation

Federal Aviation

Administration

CO
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.
Technical Report Documentation Page

1. Report No.

DOT/FAA/AM-96/22

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle

A Further Validation of the Practical Color Vision Test for En Route

Air Traffic Control Applicants

5. Report Date

August 1996

6. Performing Organization Code

7. Author(s)

H.W. Mertens, N.J. Milburn, and W.E. Collins

8. Performing Organization Report No.

9. Performing Organization Name and Address

FAA Civil Aeromedical Institute

P.O. Box 25082

Oklahoma City, OK 73125

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

12. Sponsoring Agency name and Address

Office of Aviation Medicine

Federal Aviation Administration

800 Independence Ave., S.W.

Washington, DC 20591

13. Type of Report and Period Covered

14. Sponsoring Agency Code

15. Supplemental Notes

16. Abstract

The Flight Progress Strips Test (FPST) is currently used for secondary color vision screening of applicants for air

traffic control jobs at en route centers. The test provides a practical, job-specific color vision selection criterion

involving use of color coding in the most important color task of en route radar contollers, i.e., discrimination of

the non-redundant color coding in flight progress strips (FPSs). This experiment provides a further, independent

validation of the FPST using a new criterion test. Prediction by the FPST of performance on the new and old

criterion tests was compared. Subjects were classified as normal or deficient based on anomaloscope readings. The

pass/fail cutoff score for all tests was "pass with no more than one error." All people with normal color vision

passed. Over all, for participants with both normal and abnormal color vision, the correlations between error scores

on the FPST and both criterion tests were greater than r=.93, and error scores tended to increase with degree of

color vision deficiency. The validity of the FPST was Kappa=.86 for prediction of performance on the new

criterion test, as compared to .91 for prediction of performance on the original criterion test. Part of that small

decrease in validity may be because of application of the same pass/fail cutoff score to the new criterion test, which

contains a larger number of items than the FPST. The predictive validity of the FPST was shown to be acceptably

high in this further validation with a new, independent set of actual flight progress strips as the criterion test.

17. Keywords

Air Traffic Controllers

Color vision standards

Performance tests Color vision tests

18. Distribution Statement

Document is available to the public through the

National Technical Information Service

Springfield, Virginia 22161

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

12

22. Price

Form DOT F 1700.7 (8-72)

authorized

Reproduction of completed page
A FURTHER VALIDATION OF THE PRACTICAL COLOR VISION TEST

FOR EN ROUTE AIR TRAFFIC CONTROL APPLICANTS

INTRODUCTION

Practical, "job-sample" color vision tests were developed by the Federal Aviation Administration's

(FAA's) Office ofAviation Medicine to optimize the

validity and fairness of color vision testing for air

traffic control specialist (ATCS) selection (Mertens,

Milburn, & Collins, 1995). The two practical color

vision tests, one for the en route center work option

and another for work at terminal facilities, evaluate

the ability to perform the most important, safetycritical tasks involving color coding in each work

situation. They have been implemented for secondary

color vision screening of ATCS applicants who fail

clinical color vision screening during their pre-employment medical examination by an Aviation Medical Examiner. The present research was conducted to

obtain additional evidence ofthe validity ofthe practical color vision test for the en route center work option.

The most demanding color vision task at en route

centers involves accurately distinguishing between

red and black computer printing and handwriting on

Flight Progress Strips (FPSs). The test for this task is

named the Flight Progress Strips Test (FPST). There

is at least one FPS for each aircraft that is controlled

by the en route ATCS, and the FPSs are frequently

referred to during an ATCS's time "on the boards"

(Vortac, Manning, & Rotter, 1992). The FPS is a

paper strip, 33.3 mm by 203 mm, and contains a

variety ofinformation that is either computer printed

by the air traffic control (ATC) system's computers,

or handwritten by the ATCSs. The most critical color

coding on the FPS involves the use ofthe colors black

and red to differentiate between (1) assigned versus

non-assigned (planned) information, (2) coordination versus incorrect reporting altitude, (3) north

versus south flights, and (4) east versus west flights.

The red/black color coding is typically non-redundant, especially in the critical case of distinguishing

assigned from non-assigned information concerning

altitude, route, departure, approach, fixes, clearances,

and so forth (FAA, 1989). Misidentification ofthat

critical information about an aircraft's status can

result in danger to that aircraft or place it in conflict

with another aircraft.

The FPST requires the identification of red and

black colors in reproductions of actual FPSs made by

a photographic color separation process and a lithographic reproduction process that are also used for

color-critical fine art printing. The FPS copies were

printed on the same paper used by the FAA for FPSs.

When the FPST is given, the test book is placed on a

special stand containing a light source developed to

produce a standard test illumination, with the color and

illumination level typical ofactual ATCS work stations.

The FPST was originally validated by directly comparing FPST color responses with similar responses to

a criterion test which consisted of the actual strips

from which the copies in the test were made. The

original criterion test is referred to as Criterion Flight

Progress Strips Test-1 (CFPST-1). The criterion for

passing both FPST and CFPST-1 was "no more than

one error." That criterion was based on previous

research with similar materials in which normaltrichromats made no more than one error (Mertens &

Milburn, 1992a). The predictive validity ofthe FPST,

with CFPST-1 as the criterion, was high (Kappa=.91).

The reliability ofthe FPST was also high (alpha=.93).

The high job-relevance, validity, and reliability ofthe

FPST increases the effectiveness and fairness ofATCS

color vision testing.

The present research provides additional evidence

of FPST validity by evaluating it's ability to predict

color responses in a new criterion test. This independent verification of validity also demonstrates that

FPST-based predictions of ability to discriminate

and identify colors in FPS materials can be generalized beyond the original criterion test to a larger

body of FPSs created daily in operations of the

FAA's en route centers.
METHODS

Participants

The tests were evaluated using the same, 191 people

who participated in the original validation study

(Mertens, Milburn, & Collins, 1995). Normals included 63 men and 43 women. Color deficients included 84 men and 1 woman. (The incidence of

inherited CVD is less than Vi of 1 % of all women

compared to 8 to 10% of all men.) The ages of

participants with normal color vision ranged from 18

to 58 yr with a mean age of 31.2 yr and a standard

deviation of 11.1 yr. The ages of participants with

color vision deficiency (CVD) ranged from 18 to 61

yr with a mean age of35.0 yr and a standard deviation

of 12.5 yr.

All participants had at least 20/30 visual acuity in

both near and distant vision, as measured by the

Bausch and Lomb Orthorater vision tester. Participants were recruited through advertisements in newsletters at local colleges and universities, and through

local newspapers ofthe Oklahoma City (OK) metropolitan area. All were paid an hourly wage.

FPSTand CFPST-1

The FPST included a copy of each of 30 FPSs

selected from many thousands ofstrips obtained from

six en route centers; half ofone day's output ofstrips

was obtained from each center. The 30 original strips

were copied to make the FPST, and then the original

strips were used to make CFPST-1. The 30 FPSs were

selected to be representative of the type fonts of the

two types ofcomputer printers that are used, the types

of pens and pencils used for handwriting on strips,

and representative of the range of darkness/lightness

normally found in both computer printing and handwriting in the actual work environment.

Each group of computer-printed characters in the

same line and column ofthe FPS, that did not contain

a blank space and were the same color, were considered an item on the test section pertaining to computer printing. The six strips ofthe computer printing

section had 100 items. Each handwritten altitude in

the altitude column of a strip was an item in the

section testing color vision for handwriting on strips;

there were 24 items in this section ofthe test. The FPS

copies and instructions for the test were assembled in

book form to make the FPST. Three of the 30 strips

were used for demonstration and 27 served as test

items. Respondents identified the colors red and black

in computer printing on 6 strips containing 100 items

and the color of handwriting on 21 strips containing

24 items. There was no time limit for responding and

performance was assessed in terms oftotal number of

errors and a pass-fail score. An error consisted of

misidentifying the color of a red or black item on a

strip. The criterion for passing was "no more than one

error." The CFPST-1 and FPST were essentially identical with regard to form ofthe test book, administration procedure, illumination, and scoring. The

individual strips in each of the two parts of the

FPST and CFPST-1, the computer printing and handwriting sections, were presented in different random

orders in the two tests. Because the same strips and their

copies were involved in the CFPST-1 and the FPST, the

total number ofitems was identical on those two tests.

CFPST-2

The CFPST-2 included 30 new FPSs, 3 for demonstration, 6 for color discrimination in computer printing, and 21 for color discrimination in handwriting.

Again, they were selected from many thousands of

new strips obtained from the same six en route centers.

The criteria for selection ofrepresentative strips were

identical to the criteria, given above, for preparation

of the CFPST-1. The six strips of the computer

printing section ofthe CFPST-2 had 110 items. The

21 strips in the handwriting section contained a total

of 28 items.

The same illuminant/test holder was used for all

three tests. It was designed to hold the test books in a

standard, convenient testing position, and to provide

appropriate, standard illumination for the CFPST-1,

CFPST-2, and FPST. The design is described more

completely elsewhere (Mertens, Milburn, &

Collins, 1995). A small cool white fluorescent

lamp provided an illumination level of 59 lux at

the center of the test page. That corresponded to

the average workstation illumination as measured

at two facilities, the Ft. Worth (TX) and Kansas

City (KS) en route centers.
Diagnostic Color Vision Tests

The procedure used for classification of deficiencies utilized the Nagel Type I anomaloscope to classify

individuals with red-green CVD. Other tests were

used to detect and diagnose the rare blue-yellow

deficiencies that the Nagel anomaloscope does not

detect, but none of the latter were found. These

diagnostic methods were identical to those described

by Mertens and Milburn (1992a).

Procedure

The FPST, CFPST-1, and CFPST-2 were administered in the same test session by a trained laboratory

technician. The order in which the three tests were

given was counter balanced over all participants in the

following manner. The CFPST-1 and FPST were

given with the order reversed for half of the participants. The CFPST-2 preceded them in half the participants and followed them in half the participants.

The three tests took approximately 40 minutes to

administer. Several color vision tests were given at

three other stations prior to or following FPS testing.

Those other color vision tests were given for visual

acuity screening, classification of CVD, and another

color vision study. All testing of a particular subject

was performed in a three-hour test session. Within

each session, four test periods were separated by three

5-10 min breaks.

RESULTS AND DISCUSSION

Diagnostic Classification of Participants

The number ofparticipants in each classification of

type and degree of CVD are shown in Table 1. A

complete description of anomaloscope classifications

and criteria for each is available in Mertens and

Milburn (1992a).

Representation of all categories ofred-green CVD

was obtained. The number ofnormal trichromats was

106 and the total number of deficients was 85. There

were 35 deficients who were protans (pro) and 50

deficients who were deutans (deu).

Relation of Performance on Flight Progress

Strips Tests to Color Vision Classification

Pass/FailPerformance. The CFPST-1, CFPST-2,

and FPST tests permit comparison of responses between two sets of actual flight progress strips, and

between the color copies in the FPST and two sets of

actual strips. The criterion for passing each of the

three tests was "no more than one error". That criterion was based on use ofthe similar FPS materials in

a previous experiment (Mertens & Milburn, 1992a)

in which no normal trichromat made more than one

error. The relationship of passing and failing to type

and degree of CVD is shown in Table 2.

TABLE 1

Anomaloscope Classifications of Subjects

Anomalous Trichromat

Normal Simple Extreme Dichromat

Trichromat Pro Deu Pro Deu Pro Deu

106 11 18 11 21 13 11
TABLE 2

Number of Subjects Passing and Failing on the CFPST-1,

a CFPST-2, and FPST as a Function of

Type and Degree of Color Vision Deficiency

Anomalous Trichromat

Normal Simple Extreme Dichromat

Trichromat Pro Deu Pro Deu Pro Deu

CFPST-1 Pass 106 7 11 0 3 0 0

Fail 0 4 7 11 18 13 11

CFPST-2 Pass 106 4 10 0 3 0 0

Fail 0 7 8 11 18 13 11

FPST Pass 106 7 9 0 5 0 0

Fail 0 4 9 11 16 13 11

All individuals with normal color vision passed all

three tests. Only one individual with normal color

vision made one error; that was on CFPST-1. The

probability of failing the flight progress strips tests

increased with degree of CVD. Neither protans nor

deutans who were dichromats were able to pass any

test. Only a few (three) of the extreme anomalous

trichromats were able to pass all three FPS tests, and

all were deutans. Approximately half of both simple

protanomalous and simple deuteranomalous trichromats, who have mild-to-moderate deficiencies, were

able to pass all tests. Overall, the impact of CVD on

color discrimination in FPSs seems comparable in

both protans and deutans in terms ofpass/fail scoring,

but not in terms of total errors as discussed below.

Simple anomalous trichromats had approximately a

50% chance of passing, extreme anomalous trichromats had approximately a 15% chance ofpassing, and

no dichromats were able to pass.

Error Scores. The mean and standard deviation of

total error scores and 95% confidence intervals for the

mean are shown for the three FPS tests in Table 3 as

a function oftype and degree ofCVD. The confidence

intervals are presented for comparison of means for

color deficient groups and normal trichromats. The

low, near-zero variance among normal trichromats

makes analysis of variance (ANOVA) inappropriate

for comparison ofthe means ofcolor deficient groups

and normals (Box, 1954). Error scores for the two

criterion tests and the FPST increase with degree of

CVD, and the 95% confidence intervals for the means

ofall categories ofdeficients typically did not include

the mean for normal trichromats; the exception was

the small overlap of the confidence interval for the

simple protanomalous category. That overlap was

because ofsomewhat higher variance among the simple

protanomalous on the FPST, rather than a change in

mean errors.

While the mean error values tend to be slightly

higher among the mild-to-moderate deutans (simple

deuteranomalous) than for the milder protans (simple

protanomalous), that difference does not seem of

practical significance because of the great overlap of

the distributions and confidence intervals for those

groups. In contrast, the deutans with stronger deficiencies, (the extreme deuteranomalous and

deuteranopes), make a substantially smaller number

of errors, on the average, than the strong protans

(extreme protanomalous and protanopes).

Analysis ofVariance ofError Scores

Analyses of variance (ANOVA), with normal

trichromats excluded, were performed with the

MANOVA (Multiple Analysis of Variance) program
TABLE 3

Errors on FPST and CFPST as a Function of Type

and Degree of Color Vision Deficiency

Diagnosis Mean

Standard

Deviation

95% Confidence Limits

Low to High

CFPST-1

Normal 0.01 0.10 0.01 to 0.03

Protans

Simple

Extreme

Dichro.

1.91

26.91

29.54

2.39

4.93

7.58

0.31

23.60

24.96

to

to

to

Deutans

Simple

Extreme

Dichro.

3.50

7.67

11.91

5.83

4.20

7.30

0.60

5.76

7.01

to

to

to

CFPST-2

Normal 0.00 0.00 0.00 to

Protans

Simple

Extreme

Dichro.

3.27

31.27

34.53

3.61

9.42

15.87

0.85

24.94

24.95

to

to

to

Deutans

Simple

Extreme

Dichro.

3.44

6.76

12.81

5.17

5.75

7.74

0.87

4.14

7.61

to

to

to

3.51

30.22

34.12

6.40

9.58

16.81

0.00

5.70

37.60

44.13

6.02

9.38

18.02

FPST

Normal 0.00 0.00 0.00 to 0.00

Protans

Simple 2.73 4.94 -0.59 to 6.05

Extreme 24.46 5.94 20.47 to 28.44

Dichro. 27.92 9.12 22.42 to 33.43

Deutans

Simple 2.94 5.35 0.28 to 5.61

Extreme 5.48 5.07 3.17 to 7.78

Dichro. 9.55 7.49 4.52 to 14.58
of the SPSS statistical system (SPSS, Inc., Chicago,

IL). The error scores were evaluated with ANOVA as

a function of type and degree of CVD as "between

groups" factors and test (CFPST versus FPST) as a

"within groups" factor. The assumption of homogeneity of variance was rejected by the Box-M test.

Because ANOVA is robust for violation of this assumption when the number of participants in each

group is equal, the method of compensating for heterogeneity of variance by randomly discarding cases

to achieve equal cell numbers was used (Tabachnick

& Fidell, 1989; Toothaker, 1986). While this same

technique was used by Mertens and Milburn (Mertens,

Milburn, & Collins, 1995) for comparison of FPST

with CFPST-1 in the original validation, a new,

independent random sample of participants was selected from each ofthe six groups with CVD for the

present analysis.

Each of6 groups, formed by the two types and three

degrees of CVD, had 11 cases. The main effects, of

both type and degree of CVD, were statistically significant. Protans made significantly more errors than

deutans on the FPS tests [F(l,54)=79.06, jx.OOl],

and errors increased significantly with degree ofcolor

vision deficiency [F(2,54)=46.43, /x.OOl], as discussed above. The interaction oftype and degree was

also significant [F(2,54)=23.87, /x.OOl]; there was

greater difference between protans and deutans among

extreme anomalous trichromats and dichromats, than

among simple anomalous trichromats. There was also

a significant difference for the main effect oftest, i.e.,

the within groups factor [F(2,108)=7.53, p=.001].

No interaction involving the test factor was significant. The significant main effect of test reflects the

factthat error scores tended to be slightly higher in the

CFPST-1 test than in the FPST, and slightly higher in

CFPST-2 than in CFPST-1. The latter difference may

be related to the fact that more errors were possible in

CFPST-2, because it had 14 more items. The

dichromats and extreme anomalous groups averaged

approximately two more errors on the FPST than the

CFPST. However, mean errorsforthe simple anomalous

trichromats were very similar in the two tests; the simple

protanomalous actually had a mean of 0.82 more errors

on the FPST, whereas the simple deuteranomalous had

a mean of 0.55 more errors on the CFPST. As shown

below, any small difference in difficulty between the

two tests was not reflected in pass/fail performance.

The number ofparticipants who passed both tests was

exactly the same (127), as shown in the marginal totals

ofTable 4.

Validity of the FPST

The validity ofthe FPST was evaluated by comparing pass/fail performance on FPST to pass/fail performance on the CFPST-1 and CFPST-2. Cohen's Kappa

(k), an index of agreement, was used to assess the

validity of the practical test, as recommended by the

NRC-NAS Committee on Vision (1981). The index

can be interpreted as the percentage agreement between test and criterion variable, with correction for

chance. The relationship ofpassing and failing on the

FPST, to passing and failing on both the CFPST-1

and CFPST-2 is shown in Table 4. The validity ofthe

FPST, for prediction of pass/fail performance on the

CFPST-2, was k=.86, compared to k=.91 with CFPST1 as the criterion. While k=.86 is reasonably high, it

should be considered thatthe CFPST-2 has more items.

There are two kinds of errors to consider regarding

the FPST, misses (false negatives) and false alarms

(false positives). The miss rate is the probability of

passing the FPST, given that the criterion test was

failed. Thefalse alarm rate is the probability offailing

the FPST, given that the criterion test was passed. If

the criterion for passing CFPST-2 is changed from

"no more than 1 error" to "no more than 2 errors", the

validity increases to k=.88, and the miss rate is reduced from 12% to 8%, close to the 6% value obtained with CFPST-1, the original criterion test.

Those error rates are considered acceptably low when

used to predict the criterion ofnormal versus abnormal

classification by an anomaloscope test, and are comparable to error rates ofthe better clinical color vision tests.

Reliability ofthe CFPST-2

Chronbach's alpha was computed as a conservative

estimate ofreliability, as used in previous study ofthe

reliability of the FPST and CFPST-1. Chronbach's

standardized item alpha, based on standardized item

scores, was previously reported (Mertens, Milburn, &
TABLE 4

Comparison of Pass/Fail Performance on

the CFPST-1, CFPST-2 and FPST

CFPST-2

Pass Fail

CFPST-1 Pass 122 5

Fail 1 63

Total 123 68

Kappa==0.93

FPST

Pass Fail

CFPST-1 Pass 123 4

Fail 4 60

Total 127 64

Kappa==0.91

FPST

Pass Fail

CFPST-2 Pass 119 4

Fail 8 60

Total 127 64

Kappa==0.86
Collins, 1995) as .93 for the FPST, and .95 for the

CFPST-1. The Chronbach's standardized item alpha

for CFPST-2 is .93. It can be concluded that both FPST

and both criterion tests have high estimated reliability.

CONCLUSIONS

The present research is the fourth study, in a series

ofstudies, on the ATCS color vision standard that has

addressed the need for a color vision standard, the

validity of FAA-accepted clinical color vision tests,

and the development, evaluation, and validation of

two practical color vision tests for ATCSs. The need

for practical, "job-sample", color vision selection tests

is evidenced by (1) the fact that there are some individuals with CVD who can perform the ATC color

tasks as well as normals, and (2) some common

clinical color vision tests have false alarm rates that are

too high (Mertens & Milburn, 1992a, 1992b).

Optimally, color vision screening ofATCS applicants should provide both the ability to select all

individuals who can perform safety-critical ATCS

color tasks accurately, whether they have a CVD or

not, and screen out all individuals who cannot perform those tasks accurately. A test with high predictive validity for performance in ATCS color tasks

must have both a low miss (false negative) rate to

ensure safety, and a low false alarm (false positive) rate

to ensure fairness. The most direct way ofaccomplishing that objective is to create color vision screening

tests that are "work sample" tests such that they

accurately simulate the color coded materials of the

task and the viewing conditions ofthe work environment. The stimulus characteristics that affect color

perception, such as color, color contrast, brightness,

brightness contrast, and stimulus size, must be reproduced accurately. Viewing conditions that can affect

color perception, such as intensity and color of ambient illumination and viewing distance, must also be

reproduced accurately. The practical tests should also

simulate normal work situations, rather than more

marginal observation conditions that sometimes occur, because those conditions may also degrade performance of individuals with normal color vision.

Finally, the tests should involve very simple color

identification and/or discrimination responses so that

the tests assess only color vision ability. Such tests can

be considered "practical tests" because they involve

essentially the same materials and observation conditions that are involved in the actual work tasks (Jewell

& Siegall, 1990).

The present study adds evidence ofthe high validity ofthe FPST. These results, together with findings

of the initial validation study (Mertens, Milburn, &

Collins, 1995), provide a data base supporting the

FPST which shows that it is an effective and fair

practical color vision test forscreening en routeATCS

applicants.

REFERENCES

Box, G.E.P. (1954). Some theorems on quadratic forms

applied in the study of analysis of variance problems. I. Effect ofinequality ofvariance in the oneway classification. Annals ofMathematical Statistics, 21 290-302.

Federal Aviation Administration. (1989). Air traffic

control. (FAA Order 7110.65F). Washington,

D.C.: Author.

Jewell, L.N., & Siegall, M. (1990). Contemporary industrial and organizationalpsychology. (2nd Ed.). St.

Paul, MN: West Publishing Co.

Mertens, H.W., & Milburn, N.J. (1992). Performance

ofcolor-dependent tasks of air traffic control specialists as a function of type and degree of color

vision deficiency. Report DOT/FAA/AM-92/28.

Washington, D.C.: Federal Aviation Administration, Office ofAviation Medicine, (a)

Mertens, H.W., & Milburn, N.J. (1992). Validity of

clinical color vision tests for air traffic control

specialists. Report DOT/FAA/AM-92/29. Washington, D.C.: Federal Aviation Administration,

Office ofAviation Medicine, (b)

Mertens, H.W., Milburn, N.J., & Collins, W.E. (1995).

Practical color vision tests for air traffic control

applicants: En route center and terminal facilities.

Report DOT/AM-95/13. Washington, D.C: Federal Aviation Administration, Office of Aviation

Medicine.
National Research Council-National Academy of Sei- Toothaker. L.E. (1986). Introductory statistics for the

ences, Committee on Vision. (1981). Procedures behavioral sciences. New York, McGraw-Hill.

fortesting color vision: Report ofWorking Group 4L (p. 452)

Washington, D.C.: National Academy Press. ^^ Q^ Manning> CA> mA Rotter> Aj. (1992)

Tabachnick, B.G., & Fidell, L.S. (1989). Using multi- En route air traffic controller's use offlight progress

variate statistics, New York: Harper and Row. (p. strips: A graph-theoretic analysis. Report DOT/

48-49) FAA/AM-92/31. Washington, D.C.: Federal Aviation Administration, Office ofAviation Medicine.

&VS. GOVERNMENT HUNTING OFFICE: UN -7MMI82/4WI0T