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ART Research -Note- 37-30 c pn , MC ILE COPY-'

VISUAL DISPLAY REPRESENTATION OF MULTIDIMENSIONAL SYSTEMS:

THE EFFECT OF INFORMATION CORRELATION AND DISPLAY INTEGRALITY

Elizabeth J. Casey and Christopher D. Wickens

University of Illinois

* C0

00

for

-I Contracting Officer's Representative

Michael Drillings

BASIC RESEARCH LABORATORY

Michael Kaplan, Director

DT1

FB 1 01988

U. S.Army

Research Institute for the Behavioral and Social Sciences

December 1987

Approved for public release; distribution unlimited.

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Research accomplished under contract

for the Department of the Army

University of Illinois Accession For

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ARI Research Note 87-80

.TITLe(ad ISubtitle) S. TYPE OF REPORT & PERIOD COVERED

vsua Display Representation of Multidimensional Interim Report

Systems: The Effect of Information Correlation and March 85 - March 86

Display Inteqrality S. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(e) 8. CONTRACT OR GRANT NUMBER(e)

Elizabeth J. Casey and Christopher D. Wickens MDA903-83-K-0255

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT, TASK

Department of Psycholoay, AREA & WORK UNIT NUMBERS

University of Illinois 20161102B74F

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'U.S. Army Research Institute for the Behavioral December 1987

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18. SUPPLEMENTARY NOTES

Michael Drillinns, contracting officer's reoresentative

19. KEY WORDS (Continue on revero side It necessary and Identify by block number)

Dynamic System Information

Coqnitive Psychology

Information Displays

Decision Making

120. AUSTlACT fC verse ab N nmsseey sid tify by block number)

This research note provides data on the use of object and schematic face displays to present dynamic multivariate system information. Twelve subjects

detected and diagnosed failures in a system whose variables were intercorrelated

Three visual analog displays (a bar graph display, a pentagon, and a schematic

face display) represented the system. These displays differed in the degree of

integrality of their component features. Detection performance yielded a speed/

accuracy tradeoff with little evidence of superiority for any of the displays.-

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ARI RESEARCH NOTE 87-80

20. Abstract (continued)

Diaqnostic performance showed a superiority for the more separable display,

however.

This superiority was attributed to the fact that diagnosis required subiects

to focus attention directly on a sinqle attribute, a focussinq that benefited

from a display which separated the attributes from one another. The results of

the study are discussed in a broader context of other studies which looked at

the nroximitv of information. The data also demonstrated the promise of the

schematic face disDlay as a means of displaving dynamic system information.

I

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Page I

INTRODUCTION

A visual display acts as an interface between a dynamic system

and a human operator. Its composition is critical to the performance

of the operator in controlling a system and detecting and diagnosing

system failures. As the complexity of systems has increased, the

amount of irformation available to the human operator has become

overwhelming. Therefore, there is a serious need to optimize the

display formats used to present system status information. The

operator must be presented with information in a format that requires a

minimal amount of mental transformation prior to integrating this

information with an already existing internal model of the system. The

a.. display format should also allow the operator to respond quickly and

accurately when so required.

When acting in a supervisory role, the human formulates a high

*. fidelity internal model of the system. The internal model refers to

the human operator's conception of the information structure and serves

as a basis for potential actions (Wickens, 1984). A display compatible

with the operator's internal model will minimize workload thus allowing

• faster, more accurate detection and diagnosis. The internal model may

vary along several dimensions. Two of these dimensions are the

L- frequency with which the model is updated and the degree to which the

representation of the system is spatial and/or verbal (Bainbridge,

1981; Landeweerd, 1979; Wickens and Weingartner, 1985). A third

dimension of variability is the perceived degree of integrality of the

system variables; in other words, the operator's perception of the

relative correlations between the variables or the extent to which

oL
Page 2

critical states of the system are defined by combinations of variables.

The present study examines three different methods of graphically

A representing a dynamic multiattribute system. The hypothesis to be

tested is that the integrality of system variables will be best served

by more integral displays--objects and faces--than by separated bar

graph displays.

One type of graphic representation of multivariate data which has

recently received a great deal of attention is the object display which

typically represents several variables as attributes of a single

geometric object. As an example, consider a polygon formed by

connecting the ends of invisible lines which extend out from one point

(e.g., Wood, Wise, & Hanes, 1981; Jacob, Egeth, & Bevan, 1976). The

length of the imaginary spokes, and therefore the inner angles of the

vertices of the polygon represent the values of the system attributes.

In addition to giving information about the magnitude of each variable,

the overall shape and size of this display can give insight into

relationships between the variables. A practical application of the

4integrated presentation of multivariate data is found in the field of

aviation. The contact analog display combines the two variables of

roll and pitch into a single, highly schematic representation of the

aircraft.

Some of the advantages of the object display over traditional,

separate representations of multivariate systems include subjects'

familiarity with the objects, the holistic property of object

perception by which subjects perceive the overall status of the system,

and may process the attributes of a single object in parallel (Kahneman

and Treisman, 1984; Kramer, Wickens, Goettl, & Harwood, 1986), and the

@4i" M 11 1111A~MAML6
P 3

single frame of reference against which all of the variables can he

compared. We hypothesize that the integrated representation provi~ed

by the object display will aid the operator in perceiving the

relationships among the system variables. This is because a lifetime's

experience of dealing with objects and the correlated dimensions of

these objects as they are transformed in space, allows the human

monitor to associate the integral dimensions that define an object with

a correlation between the values along those dimensions. We

%<~

444 hypothesize that this association should allow better perception of

correlated variables through integral displays.Thsypteiha

received some validation in the earlier research of Garner (Garner,

1970; Garner & Fefoldy, 1970). Other research has shown that subjects

are particularly sensitive to correlations between variables and thus a

display which optimally depicts relational information will be useful

~' C to operators of complex, multidimensional systems (Medin, Altom,

Edelson & Freko, 1982).

Several empirical studies have been conducted to assess the

-. 4 relative advantages and disadvantages of different displays. In one

such study four displays were evaluated: arrays of digits, each digit

defining a system variable; glyphs, which portrayed the system

9.., variables using the lengths of a series of rays surrounding a circle of

fixed size; polygons, the distances from the center to the vertices

representing the system variables; and schematic faces, in which each

feature delineated a system variable (Jacob, et.al., 1976). Using a

card sorting task and a paired associate learning task, Jacob, et. al.

demonstrated that people process information from standard displays

V (such as the arrays of digits) in a "piecemeal, sequential mode which
Page 4 

could obscure the recognition of relationships among t he individual 

elements". In contrast, Jacob, et. al., found that the sti uli 

represented in object displays (the polygon and particularly the face) 

are processed holistically resulting in easier detection of 

relationships among variables. 

In a series of studies conducted at the Idaho National Engineering 

Laboratory (INEL) investigators have evaluated the potential use of 

object displays as Safety Parameter Display Systems (SPDS) i :1 nuclear 

power plant control rooms (Blackman, Gertman, Gil~ore, & Ford, 1983; 

Danch a~ , 1981; Gertman, Beckman, Banks, & Petersen, 1982; Petersen, 

Smith, Banks, & Gertman, 1982). The basic functions of the SPDS 

include; alerting the operator to the occurrence of abnormal plant 

conditions, aiding the operator in identifying specific abnormal 

parameters and assisting the operator in diagnosing plant conditions 

based on the relative values of parameters. The INEL studies, which 

have evaluated different object displays in a series of tasks and with 

several different methodological techniques (psychophysical scaling, 

multivariate rating scales, checklists and decision analysis), have 

shown that generally performance with ob j ect displays i s equivalent or 

superior to that with more traditional, separate representations of 

multivariate data. Westinghouse has also proposed and evaluated an 

object display (polygon) a s one of a series of displays to be used in 

an SPDS (Little & Woods, 1981; Wood, et. al., 1981). 

Object displays have also been found useful in presenting a 

multivariate frame of reference to identify relevant physiologica l 

patterns that may delineate the seriousness of medical abnormal ities 

(Siegel, Goldwyn, & Freidman, 1971). Finally, a recent set of 
Page

II investigations carried out at Illinois have suggested conditions that

will lead to superiority of the polygon over the bar graph display.

Studies by Carswell and Wickens (1984) and Wickens, et. al. (Wickens,

Kramer, Barnett, Carswell, Fracker, Goettl, & Harwood, 1985) both

indicated that a triangle and rectangle display respectively, offered

* superior performance when three (or two) pieces of quantitative

information needed to be integrated. Another investigation by Kramer,

et. al. (1986) found that multivariate graphical information was better

integrated when it was presented as a smaller number of more integral

objects. More recently Wickens, et. al. (1985) have found that the

object display is not universally superior to separated bar graph

displays. In fact, when the task required that variables be treated

separately from each other, rather than integrated as a single unit,

the bar graph display proved superior. Similiarly the bar graph proved

to be superior when the task required that attention be focussed on one

attribute, to the exclusion of others.

Several investigators have proposed that the holistic perception

" engendered by schematic faces would be ideal for the presentation of

highly related system parameters (Danchak, 1981; Wilkinson, 1981). In

one study concerned with the facial representation of multivariate

data, the investigator found that the sterotype meaning already present

in the faces could be measured and exploited to construct an inherently

meaningful display (Jacob, 1978). Thus, in addition to the advantages

already cited for object displays, subjects' familiarity with facial

expressions appears to provide another dimension which can enhance the

perception of multidimensional data. Schematic face displays have been

found to be superior to separate numeric presentations of multivariate
Pa~c 6

information in areas as diverse as the financial profile of businesses

(Moriarity, 1979), Soviet foreign policy in Sub-Saharan Africa (Wan

Lake, 1978), the evaluation of psychiatric data (Mezzick & Vorthington,

1978), and product performance (Hahn, Morgan & Lorensen, 1983).

The program of research we describe here is concerned with

explicating the factors that influence the subject's perception,

transformation, and response to complex, multi-variate information.

This issue is pursued by investigating the conditions under which three

different displays (a schematic face, a polygon, and bar graphs)

provide an optimal representation of system status information. The

following research issues are addressed:

1) Are displays which provide an integrated

representation of system parameters (i.e. schematic

face and polygon) superior to more traditional

displays which present the same information

separately (i.e. bar graphs)? Furthermore, does tie

display format interact with the type of task which

the operator is required to perform? Some research

ehas suggested that polygons may be superior to

separated meters for detection tasks while meters

appear to be optimal for the localization of abnormal

• Ovariables (Petersen, Banks, & Gertman, 1981;

Petersen, et. al., 1982).

2) Does the correlational structure of the system

variables interact with the presentation format of

the variables? In other words, are different display

-4- % .°.- . ,% - " . .-. " ,% .% ".- . % % % % % ' N 
PIlae 7

formats optimal for systems with different

inter-variable correlations? H-ighly integrated

object displays have been proposed to be most useful

in situations in which the system parameters are

moderately to highly correlated (Wickens, 1984).

3) Do subjects wzith different degrees of spatial ability

adopt different strategies to perform detection and

diagnosis tasks? Can we optimize the subjects'

performance by presenting system status information

* in a manner consistent with the subjects' preferred

processing strategy?

'5 In this experiment, a temperature process monitoring task was

simulated in which subjects were required to detect and then diagnose

J. system failures. Three different displays were evaluated, each

representing the correlated variables of the dynamically changing

system. The displays, in increasing order of feature integrality were

a bar graph display, a polygon, and a schematic face (Wickens, 1984;

Jacob, et-al., 1976). The monitored system was presented at two levels

of inter-variable correlation, and the spatial/verbal abilities of the

subjects were measured.

This task required the integration of the system information:

subjects were required to attend to the pattern of correlation between

the variables rather than determining whether any of the variables

exceeded Bpecified levels of normality. Therefore, it was expected

that failure detection and diagnosis performance should increase with

display integrality. This effect was predicted to be stronger for the
Page 8 

system with the higher correlation between variables. 

HETHOD 

Subjects 

Six male and six female University of Illinois graduate and 

en ineeri~g students participa ted in the study . The subjects, all of 

whoo \-Tere right handed, ranged in age from 19 to 25 years. They earned 

a base rate of $3.50/hour for all four meetings, $.50/day for arriving 

on time , and bonuses ba sed on performance during the two experimental 

sessions. 

Task 

In a simulated setting, the subj ect monitored a display of the 

temperatures in five chambers to determine the status of a heating 

system, as shown schematically in figure 1. The temperature i n each 

chamber was controlled by two sources: a general global furnace which 

served all five chambers equally, and a local space hea t e r withi n each 

chamber which allowed different thermostat settings. The global 

furnace provided most of the heat (represente d as the s igna l a t t he t op 

of figure 1 and as S in the tempera ture equation below each chamber in 

the same figure) so that the temperatures between the five chambers 

were correlated over time. However, due to di fferences in insulat ion, 

local temperatures heating needs, e tc. n~ in the figure), the 

correlations between the chambers were not perfect, as each space 

heater added its own "noise." to the total temperature variation. A 
Signal

2. FFurnace

II

rloom 1 Partial Room 5

Blockage

Space IL H I Heater I

Room 2 Room 4

S+N1 S+N 5

LLTHJ L!HT 11

S+N 2

S+N3

Figure 1. The heating system monitored by subjects. During normal

operation the furnace provided the "signal" heat to each of the five

chambers. A Space Heater (SH) in each chamber provided additional

heating input to the chamber's temperature signal. The curves below

the chambers are examples of each chamber's temperature variations

over time. During a failure, partially blocked ducting leading to

one of the chambers decreased the influence of the furnace input on

the chamber's temperature.

9
failure occurred when a pipe leading from the global furnace to one of

the chambers became partially blocked requiring the chamber's spa-ce

heater to bear a greater burden in heating the chamber. As a result,

the temperature in the blocked chamber correlated less with the

tempratresof the other chambers than it had before the failure. An

example of this change in correlations is demonstrated at the point

marked "failure"' in figure 2. The temperatures of the other chambers

continued to correlate with each other over time.

The subject was instructed to detect failures as quickly and as

accurately as possible by pressing a button when the correlation of

changes in one signal with changes in the others appeared to drop.

Immediately upon correctly detecting the failure, or being told that a

failure was missed, the subject diagnosed the location of the failure

by pressing a button corresponding to the chamber whose temperature had

become less correlated.

Displays

Three visual, analog displays were used to represent the system.

% In figure 3 temperatures of five chambers are shown using all three

displays. The traditional bar graph represented temperatures by the

height of the bar with each bar corresponding to one chamber. The

object display was a pentagon which represented a chamber's temperature

by the distance from the center point to one vertex. Thus, five equal

and low temperatures formed a small, equilateral pentagon. The third

display was a schematic face. Each of five features represented the

temperature in a chamber: ear length, eyebrow angle, eye length, nose
V1

V2

V3

".

V4

V 5

O.1

Time

AFigure 2. Demonstrated here are representative examples of the

temperature of each chamber as they vary over time. The output Ssignals are correlated with each other in the beginning. From the

time the failure occurs (in chamber V2 in this case) the correlation

of the temperature in the chamber with partially blocked ducting

decreases with respect to the temperatures of the other chambers.

The correlation of the other four chambers with each other remains

constant.

11

O,
Display Type All Low All High Failed Mode

Bar Graph ---

Pentagon

FaceAA

I,.I

Figure 3. The three displays used to represent the system information.

The first two columns of displays are examples of the features with

equal temperatures at low and high levels. In the third column the

temperature in one of the chambers is lower than the other

temperatures. Note that failures were detectable only by

discovering changes in the pattern of movement, the change. in

correlation, between the five display features. Differences in

absolute levels of the features at one instant in time were expected

under normal operation and did not by themselves indicate system

failure.

12
Page 13

length, and mouth curvature. As shown in figure 3~a sad face with long.

features represented all high levels of the variables and the face

* looked happy (or possibly devious) with short features when the

* variables were all at low levels. Thus an effort was made to allow the

features to correlate over time in the normally operating system, in a

manner consistent with the feature correlations over time caused by

changes in emotional expression.

displays was adjusted in order to insure equal discriminability on the

basis of results from a preliminary psychophysical scaling study. This

study is described in the appendix.

System Dynamics

The displays were updated every half second with temperatures at

fifteen discrete levels. The temperature in chamber i was determined

using the equation:j

Temp(i) -Global + Deviation~i) + Error~i)

Global was the contribution of the global furnace. Its value was

determined by a random walk, and was the same for each chamber.

Sampling from a uniform distribution, the computer determined whether a

change from the previous level would occur and, if so, the magnitude of

a change using the following probabilities:

no change 30%

N 1 level 40%

2 levels 20% z.half increase, half decrease

3 levels 10%

Each chamber deviated from the global temperature by a constant
~..~ -~Page 14

value, Deviation(i), for the chamber. This value represented

differences in thermostat settings. It ranged from -1 level to +1

level between the five chambers. (Niote, in the two "normal" columns of

figure 3, the offset deviations are not reflected.)

Error(i) represented the fluctations from moment to moment that

were due to differences of insulation, location, etc. For each chamber

and each display update the value of Error(i) was chosen randomly from

a range of values. Two different baseline levels of correlation were

employed; these were established by the magnitude of the error~i),

* relative to the global signal. The range for the highly correlated

system was from -1 to +1 except for the failed chamber whose range was

from -3 to +3. For the system with the lower correlation the values

ranged from -2 to +2 with fluctuations of the failed chamber ranging

from -4 to +4. Operationally, these values produced the mean

correlations between system variables of .98 (high) and .93 (low)

during normal operations, and correlations of the failed variable with

the others of .89 (high) and .78 (low).

When the sum of the three values contributing to a chamber's

temperature was less than one or greater than fifteen, the displayed

temperature was set to zero or fifteen.

Failures occurred at a random time within an interval of ten and

fifty seconds after the beginning of a trial or after diagnosis of the

most recent failure. Selection of the chamber whose pipes would be

blocked was also random.
V-% %-. Nr.

Page 1

Apparatus

The displays were generated by a PDP-11/44 computer on a

Hewlett-Packard 1310A display in a small, darkened chamber. The three

displays subtended approximately the same degree of visual angle--50

high by 6.5 wide. The lap-held response board featured a stationary

joystick with a thumb button for detection response and a five-button

box on the right hand side for diagnosis response. The five-button box

had four buttons arranged in a semi-circle on top and a fifth button on

the front of the box. This arrangement was one that produced an

stimulus-response compatible mapping between display features and

buttons for both the bar graph and pentagon displays. The mapping was

less compatible for the face display. However, a pictorial

representation of the appropriate facial feature was presented above

-* Jeach button. The circular pattern was designed based on measurements

taken from ten people with a wide range of hand sizes. Data were

recorded on magnetic tape.

-d'

Procedure

All subjects participated in four sessions: a testing session, a

Atraining/practice session, and two experimental testing sessions that

were identical with the exception that the order of presentation of the

displays and correlation levels were counterbalanced across subjects.

%

The first meeting was a group sessions which lasted for one hour.

The subjects were given four tests of spatial and verbal abilities.

The second meeting was a two-hour training session beginning with a
Page 16

detailed verbal, aind pictorial description of the svstem and task.

W~hen the subject had a good basic understanding of the experimetnt,

training began in performing the task.

The three types of on-line training were three-minute, modified

versions of the actual task. In the first version the word FAILURE

appeared on the screen as soon as the system failed followed by an X

above the failed chamber. The second type of training allowed the

subject to cause a failure by pushing the button corresponding to the

chamber whose pipes would be blocked. These two versions gave the

subject the opportunity to observe the system in the normal mode to

learn the types and amounts of its variation to expect in the system,

and then to compare the differences that arose when the system switched

to failed mode.

During the third version of training, the subject learned to

perform the task without the cues of the first two training types.

This was identical to the procedure that would be employed during the

experimental session. At each level of training the subject exercised

* with each display at both correlation levels for a total of 72 minutes

of on-line practice.

The third and fourth sessions were the experimental sessions which

lasted about two hours each. On each day the subject performed with

one correlation level and all three displays with three ten-minute

blocks per display. Each block consisted of between eleven and

eighteen trials which were structured as shown in figure 4.
Page 17

detection-->I diagnosis-->I _ _ _ _ _ _ _ _ _ _ _ I _ _ _ _ I _ _ _ _ _ _ _

I < ---- 10-50 sec ---- >1< ------1 0 sec ----- >1< ------ lOsec ------ >1

Begin System Begin Begin

Trial Failure Diagnosis New

Trial

Figure 4. The time course of events for a single trial. In every

trial the system operated normally for at least ten seconds. From

that time, a failure could occur at any moment. A failure always

occurred within fifty seconds of the beginning of the trial. When a .1 failure began subjects had ten seconds to detect its occurrence. 3,- Subjects had another ten seconds from the time of detection (or from

the end of the ten second detection interval) to diagnose the

failure. A new trial began after the diagnosis was complete or when

the ten seconds diagnosis interval was over.

RESULTS

*' Statistical analyses were conducted on four dependent measures:

two for detection and two for diagnosis. Performance on the detection

task was measured using average time to detect the failure (measured in

milliseconds) and the A-prime measure of sensitivity, a non-parametric

measure of signal detection theory which incorporates the probability 0

of a hit and the probability of a false alarm. A-prime was computed

for each block of trials using the formula:

,* A' I - 1/4 ( [P(FA)/P(H)] + [ (I-P(H)) / (I-P(FA)) I }

(Wickens, 1984) where P(H) - #hits / #trials and P(FA) - #false alarms

/ #false alarm intervals. A false alarm interval lasted ten seconds

since a failure could occur anywhere from ten to fifty seconds into the

trial. The number of false alarm intervals in a block was determined

0 a
Page 18 

to be the total time a signa l was not presented divid ~ be t en (the 

length of a false alarm interval). 

Diagnosis performance was measured using average time to diagnose 

the failure (to the nearest millisecond) and the probability of a 

correct diagnosis. 

All four dependent measures were calculated for each block. These 

summarized values were analyzed by means of a three-way repeated 

measures ANOVA. The three within-subject factors were display (bar 

graphs, pentagon, and face display), correlation level (high vs. low), 

and block number ( 3 replications). (Initially subjects were grouped by 

spatial/verbal abilities, but preliminary analysis revealed tr.at this 

factor was not a significant source of variation on any of the 

measures. Hence, spatial/verbal ability was excluded from subsequent 

analyses, and data for all subjects were pooled.) 

The graph at the top of figure 5 illustrates the speed and 

accuracy failure detection performance ~easure for each display, cross 

plotted at each correlation level. The display symbol is positioned at 

the average levels of the dependent measures collapsed a c ross 

correlation levels. In this representation, "good" performance (rapid 

and accurate) are to the upper left, "poor" performance to the lower 

right, while shifts in a speed versus accuracy set are represented by 

movement along the positive diagonal. The display manipulation 

produced a main effect on failure detection latency (£(2,22)=6.3, 

p=.0069). On the average, subjects detected failures almost half a 

second faster with the face display CM=3.046 sec) than with the bar 

graphs (~3.496 sec) or the pentagon (~=3.476 sec). Accuracy in 

detection on the other hand was generally highest with the bar graph 
.%s. - rrr W 'V WZ W XW - - V - - - - -- - - - -

1.00 I , , ,

Good DETECTION

.95-

S ~.9o/7

,.85/

.80- Correlation

S0 High

0 Low Poor

2.8 3.0 3.2 3.4 3.6

Latency (sec)

100 I 1

SGood DIAGNOSIS

90\

" 80

* 0

70-

'" p Poor]

I II

1.5 2.0 2.5 3.0

Latency (sec)

%'Figure 5. Detection (top graph) and diagnosis (bottom graph)

performance results. In both graphs accuracy is plotted against

latency for each display at each system correlation level. The

endpoints of each line represent high and low correlation levels.

The display symbol is positioned at the average of the dependent

measures collapsed across correlation levels.

0, 19
P3ge 20

(M=0.919) and lowest with the face display (M=0.849)

(F(2,22)=20. ,p=.0000).

The Scheffe t-test was used to determine whether performance was

better using the bar graph than on the pentagon at the .05 significance

level. There were no differences between the two displays in the

latency of either detection or diagnosis. However, subjects did

perform more accurately on the bar graph when detecting failures

(t411)=2.4, p<.05) and when diagnosing the failures (t(1l)=4.7, p<.05).

It is apparent that the effect of the correlation manipulation was

to produce a shift in the speed/accuracy tradeoff, with performance

being faster (F(1,11)=4.7, p=.0 5 28 ), but less accurate (F(1,11)=47.4,

p=.0 000), with a lower correlation between variables.

Both dependent measures of diagnosis performance, shown in the

lower graph of figure 5, varied reliably as a function of the display

being used. Most of the latency difference was contributed by the face

display (M=2.766 sec) which took over 600 msec longer to diagnose than

the bar graphs (M=2.019 sec) or the pentagon (M-2.139 sec),

(F(2,22)=9.2, p-.0013). The probability of a correct diagnosis was

highest for the bar graph display (M-0.887), slightly lower for the

pentagon (M=0.846), and another ten percent lower for the face display

(_M-0.742), (F(2,22)-22.7, p-.O000).

Correlation level affected both accuracy and latency: higher

correlations produced performance that was both faster (F(I,11)=7.7,

p-.0180) and more accurate (F(1,I1)-45.5, p-.0000).

As shown in figure 6, response time in both the detection task and

the diagnosis task did not change significantly as a function of
1.00

Good DETECTION

.95

.90 2 3

"") .85

.80

Poor"'

, 3.2 3.3 3.4 3.5

* Latency (sec)

100 1 ,

Good DIAGNOSIS

95-

'.90

0

P1.

0

',%Q... 8 5 -

53 2

Poor]

21
Page 22

practice. Ilowever, the sensitivity measures for both types of tasks

were improved v.ithi experience. A-prime, the measure of sensitivity for

detection, increased with each block (M(1)=.867, M(2)=.887, M(3)=.894)

(F(2,22)=4.9, p=.0 17 8 ) as did the probability of a correct diagnosis

K (M( L=.SOI, M(2)=.836, M(3)=.833) (F(2,22)=5.7, p=.0101).

There were no significant interactions for any of the measures.

* DISCUSSION

The assum-ption that subjects needed to attend to the patterns of

correlations between the variables to accomplish the failure detection

and diagnosis tasks led to the hypothesis that performance on both

tasks would be superior with the more integrated displays. However,

the hypothesis did not prove to be correct.

Detection performance showed a speed/accuracy tradeoff on all

three factors: display type, correlation level, and practice.

Detections using the schematic face display were faster and less

% accurate than the other two displays. The least integrated display,

* the bar graph display, afforded the most accurate but slowest detection

responses.

The lower correlation system, which seemed more difficult to the

S. subjects, yielded faster but less accurate failure detection than the

more correlated system. The speed/accuracy tradeoff even held for the

block factor. With increasing experience, failure detection became

more accurate but did not become faster. Generally, with increasing

experience, increasing system correlation, and decreasing display

integrality, subjects seemed to stress accuracy over speed in their
Page 23

responses in detecting failures.

Diagnosis performance, however, revealed a different pattern of

results. In this case for every factor the difference in performance

4 at each level was in overall quality--there were no speed/accuracy

tradeoffs. The more integral the features of the display were, the

slower and less accurate was performance. The higher correlation level

afforded better overall diagnosis performance, and with increasing II experience subjects responded more quickly and more accurately.

Because detection and diagnosis show qualitatively different

patterns of effects from each other for each of the two manipulated

variables, and because the two tasks also manifest different kinds of

information processing routines, each will be discussed in turn, before

* presenting a general theoretical framework for interpreting the effects

of display integrality.

As noted, the data for the detection task indicate a

speed/accuracy tradeoff across both the display manipulation and the

manipulation of correlation. Some conditions (face display, low

correlation) yielded fast but inaccurate responses, while others (bar

graph, high correlation) yielded responses that were slow and accurate.

Two interpretations however may be offered to this pattern of data. On

the one hand, it is possible that there is really little difference in

the efficiency or effectiveness of detection performance across these

conditions. They differed only in the "set" for speed versus accuracy

with which subjects chose to operate. On the other hand, there may

have been some fundamental limitations of the less accurate conditions

(low correlation and face display) that prohibited subjects from

attaining a higher level of accuracy, even by prolonging latency. Such

04
S..

Page 24

a phenomenon has been observed elsewhere in signal detection

experiments that have shown negative speed/accuracy tradeoffs (i.e.

less accurate performance with longer latencies; Vickers, 1970;

Welford, 1976). Hence, in these "difficult" conditions subjects may

- .4~have decided that since there is no advantage to accuracy to be gained

by waiting longer, a rapid response might as well be given.

The critical test necessary to choose between these two hypotheses

would have been to induce subjects to adopt different speed/accuracy

sets within a condition. Thus, according to the first hypothesis, a

request for subjects to adopt a conservative criterion setting (slower

and more accurate), in the face (or low correlation) condition wo'ild

have "moved" performance to a level at which it had the same speed and

accuracy as the more integrated displays (and high correlation

conditions). According to the second hypothesis, the request would

have prolonged latency, with no increase in accuracy. Unfortunately,

since this manipulation was never performed, the hypothesis cannot be

tested. There is, at most, some converging evidence for the second

hypothesis, based upon the subjective reports that the face display and

low correlation conditions were somewhat more difficult.

Unlike detection, there was no ambiguity regarding the relative

ordering of merit of diagnosis across conditions. The lower

correlation condition and the face display were reliably worse along

both performance dimensions. The polygon display was reliably less

accurate than the least integral bar graph display.

An overall summary of the results of both tasks would state that

more integral displays (the polygon and particularly the face), lead to

poorer diagnosis performance, but have a less harmful effect (or
Page 25

perhaps no effect) on detection performance. This result, which for

resuts i the literature that have obtained an advantage for the

objet orface display in diagnosis sorts of tasks (e.g., Jacob, et.

al. 196;Carswell & Wickens, 1984; Wickens, et. al.., 1985). The

difference between the present result, and those that have found

superiority of integral displays appears to lie in a difference in the

P nature of the diagnosis required, and in the contrast between focussed

attention and information integration. In both the paradigms used by

Jacob, et. al. and by Carswell and Wickens, there was a complex mapping

of variable values to diagnostic state. That is, each diagnostic state

required the simultaneous consideration of more than a single variable.

This condition of information integration is one that we have argued

elsewhere, is best served by integral displays (e.g., Carswell &

Wickens, 1984; Wickens, et. al., 1985; Wickens, 1986). In contrast,

the diagnosis task used in the present experiment imposed a 1-to-i

mapping between variable and diagnostic state, therefore imposing more

% of a requirement to focus attention on a single variable; that is,

treating the failed variable as separate and unique from its neighbors.

* It would be plausible to argue here that such conditions are not

* favored by integral displays. In fact this view is consistent with

another condition reported by Wickens, et. al., (1985) in their

comparison of bar graphs with object displays. When their task was

modified to require a separate 1-to-I mapping for each variable to a

different response, the integrality advantage was actually reversed,

and performance became superior with the bar graph display. The

results of Peterson, et. al.'s (1982) investigation comparing separated
Page 26

meters with integrated stars also are somewhat consistent. When fault

location was required (a 1-to-i mapping), performance was best with the

separated meters, and poorer with the integrated star, although they

found that the separated bar graph display did not perform at the level

of the meters.

Viote that the interpretation offered above is consistent with the

results of the detection task in the current data as well as those of

%.I. Peterson, et. al. (1982). Detection, like the diagnosis tasks of

Carswell and Wickens, requires a many-to-i mapping, in which a larger

number of displayed variables must be mapped into a smaller number of

cognitive and response states. This condition is by definition one of

information integration which should benefit from the more integral

displays. In the investigations of Peterson, et. al. and Carswell and

Wickens, this benefit was present. In the current study there was no

absolute benefit to the integral face and object displays in detection

-only a reduction of their cost, relative to the diagnosis condition.

The absence of an absolute benefit for object displays in the

present detection data could probably be attributed to some other

inherent disadvantage to the face and polyon display in the current

study. Subjects, for example, voiced some complaint that the large

visual angle subtended by the moving parts of the polygon display was

fatiguing. Also the selection of features for the face display may not

have been optimal. On the one hand, two of the features chosen, ear

length and nose length, are not features that naturally change

dynamically within a face. A second possible source of incompatibility

with the face display relates to the concept of heterogeneity. The
4

Page 27

features of the face, while holistically integrated, are also

heterogeneous, each one having a different physical appearance,

meaning, and emotional content. This heterogeneity, of course, is in

contrast to the points on the pentagon, or the five bar graphs which

are homogeneous. Yet, the particular system which provided the context

for the scenario was also a homogeneous one, with all five variables

having the same semantic meaning. Thus, a "compatibility of

homogeneity" between display and system variables that was present for

the pentagon and bar graph display, was absent for the face display.

Both of these issues, the role of constant versus changing features in

the face, and the issues of homogeneity, are presently under

investigation in follow-on studies in our laboratory.

The broader context of display integrality and information

processing, within which the current results may be interpreted is

presented in figure 7. On the ordinate of the figure is represented

what is termed a "Display Proximity Advantage" or D.P.A. That is, an

advantage in a particular experiment for displays that are "close" or

integral, such as the face or the object, over displays that are

separated. Negative values of this D.P.A. are those such as observed

for the diagnosis task in the present experiment. The two dimensions

of the abscissa in this three-dimensional representation reflect the

two task/cognitive variables that were hypothesized to influence the

YD.P.A.: information integration and information correlation. To the

extent that a D.P.A. is modulated by either of these cognitive

variables, the concept of display integrality moves beyond the

perception domain, and makes it necessary to invoke central processing

concepts such as the internal model or mental integration. This is
-t-3 4

2

i)

Integration5

Display

..... .. .-:: : - ::..::.. 

P x im ro ityo ........ ............

Proximity, , !:/

Advantage ....... /

0 " t'5 " ..................... ...... .. ......... .

Correlation

7

1. Barnett (Wickens et. al., 1985, exp. 5).

2. Banks et. al., 1982.

3. Carswell & Wickens (Wickens et. al., 1985).

4. Jacob et.al., 1976.

5. Garner, 1971.

6. Kramer et. al., 1985.

7. Casey & Wickens, present exp.

* 8. Goettl (Wickens et. al., 1985, exp.2).

Figure 7. This figure portrays the Display Proximity Advantage

(D.P.A.) as a function of the amount of correlation between the

displayed values, and the degree to which those values must be

integrated. Conditions for which integration is low are those that

require either focussed attention on one source of information, or

independent processing of several sources (i.e., dual or multi-task

processing). Each experiment is designated by a number, identified

in the legend ahcve. Solid lines and planes indicate a Display

Proximity Advantage, and thus lie above the plane of the surface.

Dashed lines and open planes indicate experiments or conditions with

a disadvantage to proximate displays. They thus depict negative

values below the origin of this three-dimensional representation.

28
Page 29

because a display principle that is purely perceptual in its

characteristics, should be unaffected by the later cognitive processes

required of the displayed information. The relevance of the two

abscissa dimensions to cognitive, rather than perceptual phenomena are

as follows: task integration requires that the joint consequence of

all stimuli must be taken into account before a response is made --

that is, some sort of mental operation must be carried out on the

stimulus as an ensemble, rather than either focussing attention only on

.1. a single stimuli, or dividing attention between stimuli but processing

each independently of the others. Stimulus correlation need not have

cognitive implications. However, to the extent that correlated stimuli

are processed better than orthogonal stimuli (a valid assumption in

decision-making research; Moray, 1981; Ebbeson & Konecki, 1981), then

some higher level of cognitive processing must be operating to extract

the presence of this correlation, and thereby use it to advantage.

Figure 7 then shows the effects of task integration and

correlation on the D.P.A. Each vertical line on the graph indicates a

e pair of conditions in which display proximity has been manipulated, by

q one form or another. A vertical "slice" is an experiment in which

display proximity has been manipulated orthogonally with another

variable that effects either the degree of correlation, or the amount

of integration required. A vertical solid has manipulated both

proximity and correlation orthogonally. For example, since the current

experiment varied the degree of integration between detection (higher)

and diagnosis (lower) using information sources which were always

correlated (but whose correlation varied), this experiment is

4" represented by a solid whose position on the plane is as labelled.
' 4 

Page 30

Some experiments have contrasted the D.P.A. in two conditions that have

varied, in a confounded manner, both in the correlation between their

* -~ inputs and in the amount of information integration required. Hence,

the "planes" defined by their results are oriented at an angle to the

two axes.

While it is difficult to draw any conclusion with absolute

certainty from the data shown in this representation, two general

trends appear to be noteworthy. (1) There is a general tendency for

the D.P.A. to increase (or a display proximity disadvantage to

dissipate), as tasks require more information integration, or less

divided and focussed attention. That is, the contours "slope" upward

from the front of the figure to the back. (2) The effect of

correlation on the D.P.A. appears to be substantially less. There is

little trend in D.P.A. from the left of the figure to the right as

correlation between displayed variables increases.

In summary, it is hoped that this representation will provide the

%4 foundation for a theory-based means of predicting the circumstances in

V: which more "integrated" displays may or may not be employed to

M. advantage over more separated formats. Such guidelines will not,of

course, be absolute. For example, as we have noted in the present data,

there may be numerous influences on the merits of a face display that

exist independent of the degree of integration required (e.g., the

On*

assignement of features to variables or the heterogeneity of

variables). However, such a framework does, it is hoped, establish the

foundation for a theory of display integration.
APPENDIX

Scaling Study

It is important in any comparison of visual displays to determine

whether the superiority of a given visual display can be accounted for

by perceptual factors. Therefore, in order to properly compare these

displays we first had to ensure that it was equally difficult to

perceive a change in a system variable regardless of the display or

display feature on which that variable was represented. Thus, a

psychophysical scaling study was conducted on five displays.

(Originally meters and glyphs were being studied in addition to the

other three displays. These were omitted in the correlation/display

study because it proved impossible to perform the task in that study

with those displays.)

Ten college students participated in the experiment. All were

right-handed with normal or corrected-to-normal vision. During the two

hour session the subjects performed the task with each of the five

displays. The subject's task was to decide whether two sequentially

presented displays matched or mismatched. The importance of both speed

and accuracy was emphasized. Subjects pressed one response button if

k4 the displays matched and another if they did not. In a single block

of trials only one of the five variables on a display was to be

attended by the subject. The other four variables remained at constant

levels. Each of the five features of the face varied in different

blocks.

For each display and feature, ten equidistant levels were defined.

The "standard" (SI) was always either at level 5 or at level 11. The

31
comparison stimulus (S2) varied as follows:

STANDARD PERCENT COMPARISON PERCENT

TRIALS TRIALS

UP TWO LEVELS 12.5

LEVEL 5 50.0 Up ONE LEVEL 12.5

OR 0 SAME 50.0

LEVEL 11 50.0 DOWN ONE LEVEL 12.5

DOWN TWO LEVELS 12.5

In total there were nine blocks, five for the face and one each

for the other four displays. Before each block of 160 trials, the

subjects had fifteen practice trials. Experimental blocks and response

buttons were counterbalanced across subjects.

The amount of time required to decide whether two displays matched

or mismatched was affected by the type of display being judged

(F(8,72)-4.3, p<.01). The order of displays from fastest to slowest

was the meters (M-318 msec), bar graphs (M=334 msec), polygon (M342

msec), glyphs (M-373 msec) and schematic face (M=394 msec). The amount

of time required to compare different facial features ranged from 375

*msec for the eyebrows to 417 msec for the mouth. RT was also

influenced by stimulus level. Slight mismatches took longer to respond

to than matches and more obvious mismatches (F(4,36)-18.3, p<.01).

There was an interaction between display type and stimulus level such

that RT performance with meters and bar graphs was not differentially

affected by stimulus level (F(32, 288)=1.7, p<.01). Error rate

generally followed the same pattern as RT with larger error rates being

associated with longer RTs.

The results of this scaling study were used to adjust the

32
magnitudes of the physical changes of the display components. The

ranges of variations were scaled to be psychophysically equivalent.

Thus, any differences in performance among displays are not

attributable to perceptual factors.

3

,

4

4 33
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