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ENG PEERING-PSYCHOLOGY RESEARCH LABORATORY

University of Illinois at Urbana'Champalgn

TECHNICAL REPORT EPL-84-2/NASA-84-2

OCTOBER 1984

The Dissociation of

Subjective Measures of

Mental Workload and

Performance

FINAL REPORT

Yei-Yu Yeh

Christopher D. Wicken

dj

Prepared for

National Aeronautics and Space Administration

NASA Ames Research Center

Moffett Field, CA

Contract No. NASA-NA-G-2-169

Approved for Public Release: Distribution Unlimited
Table of Contents

page

Abstract iii

Introduction 1

The muftiple resources model 2

Subjective Workload ... . 4

The structure of perceived workload: A Multidimensional

Scaling approach 5

A model of verbal report data 6

A theory of dissociation between performance and subjective

workload ; 8

Experimental Overview 12

Hypotheses of the dissociation 13

Secondary issues .... . 17

Methodology of the analysis 19

Test of dissociation . 20

Workload measures in comparable units 21

Test of the resource demand of a task: The additive factors

methodology 23

Experiment 1 26

Method 27

Subjects 27

Apparatus . . 27

Tasks 27

Procedure 29

Experiment 2 30

Method 31

Subjects 31

Tasks 31

Procedure 33

Experiment 3 34

Method . 35

Subjects 35

Tasks 35

Procedure 36

Results 37

The processing demands of various tracking tasks . . . . . . . . .37

Experiment 1 37

Experiment 2 41

Experiment 3 44

Rating reliability 46

Predicting global workload from specific rating scales 47

The perceived structure of subjective difficulty 48

Dissociation between performance and subjective measures .... . 51

Perceptual vs. Response resources 52

The number of display elements 54

Single task difficulty vs. dual task competition 56

Dual task competition for common vs. separate resources ... . 58

The effect of resource-investment factors 61
page

Discussion 64

MDS results . . . 67

Discussion of dissociations 69

Strength of dissociations 77

Subjective workload scales . 79

Conclusion 84
m

Abstract

A dissociation between performance and subjective workload measures

occurs, when two task configurations are compared and one shows better

performance, but is perceived as subjectively more difficult than the other.

The dissociation phenomenon was investigated in the theoretical framework of

the multiple resources model. Even though the underlying structure of

subjective workload strongly corresponds with the structure of processing

resources, subjective measures do not preserve the vector characteristics in

the multidimensional space described by the model. A theory of dissociation

(Wickens & Yeh, 1983) was proposed to locate the sources that may produce

dissociation between the two workload measures. According to the theory,

performance is effected by every aspect of processing whereas subjective

workload is sensitive to the amount of aggregate resource investment and is

,• dominated by the demands on the perceptual/central resources. The proposed

theory was tested in three experiments, employing different combinations of

a tracking task and a Sternberg memory search task.

In supporting the theory, the results showed that performance improved

but subjective workload was elevated with an increasing amount of resource

investment. Furthermore, subjective workload, being affected by the

aggregate demands, was not as sensitive as was performance to differences in

the amount of resource competition between two tasks. The demand on

perceptual/central resources was found to be the most salient component of

subjective workload from both the multidimensional analysis of the hidden

structure and the regression analysis of the underlying components.

Dissociation occurred when the demand on this component was increased by the

number of concurrent tasks or by the number of display elements. However,
IV

in contrast to the prediction, demands on response resources were weighted

in subjective introspection as much as demands on perceptual/central

resources. The implications of these results for workload practitioners are

described.
Introduction

The concept of mental workload is of paramount importance in designing

man-machine systems. The main concern of a designer is to enhance

performance by moderating the workload experienced by the operators.

Numerous techniques have been proposed to study and define the concept of

mental workload. Wierwille and Williges (1978) classified these techniques

into three categories: performance, physiological, and subjective

assessment.

Despite the fact that all of these techniques are reputed to measure

the same hypothetical construct, they provide different workload values in

many conditions. Given the same manipulations, one technique may indicate

an increase whereas another technique may indicate no change or a decrease

in workload (e.g., Hicks & Wierwille, 1979; Williges & Connor, 1983). Such

dissociations lead to various operational definitions of mental workload.

Even though there is no consensus with regard to the definition of

mental workload, it is agreed that workload is multidimensional. Mental

workload can only be specified as a vector in a to-be-defined space

(Johannsen, Moray, Pew, Rasmussen, Sanders, & Wickens, 1979). To define the

multidimensional space of mental workload, the underlying dimensions must be

identified. Two models have presented candidate dimensions to define such a

space.

The model proposed by Sheridan and Simpson (1979) has been accepted by

many researchers as a framework for assessing subjective workload.

According to this model, workload is defined by three descriptive

categories: (1) task time constraints, (2) task uncertainty and complexity
of planning, and (3) psychological stress. However, these three dimensions

were originally chosen through an intuitive subjective task analysis and the

theoretical basis was not provided. Although these scales have undergone

further development and refinement (Reid, Schingledecker & Eggemeier, 1981),

it is still not entirely clear how these dimensions relate to human

information processing and performance. It is also uncertain whether each

dimension is itself multidimensional and how the three dimensions relate to

each other (Boyd, 1982).

o

The multiple resources model

As an alternative approach, the dimensions of mental workload may be

defined by the structure that characterizes human information processing and

performance. According.to this approach, workload is multidimensional

because the human information processing system is muitichanneled (Sanders,

1979) and depends upon separate resources of limited quantity (Navon &

Gopher, 1979). Wickens (1981, 1984) identified three dichotomous dimensions

as the functional composition of this processing system. These dimensions

are (1) stages of processing (perceptual/central vs. response selection and

execution), (2) modality of input (visual vs. auditory) and of output

(manual vs. vocal speech), and (3) codes of central processing (spatial vs.

verbal).

A hypothetical structure of these dimensions is shown in Figure 1.

However, these dimensions may not be organized in such an orthogonal manner

but in a hierarchical combination (Wickens, 1984). Each cell in the space

may be conceived of as a semi-independent subsystem. Resources are shared

within each subsystem but are not transferable to another subsystem of the

same dimension. For instance, perceptual/central resources cannot be
exchanged to compensate for a deficit in the response resources. When two

tasks compete for the same resources within a subsystem, resource

competition occurs and a deficit is shown in the performance. When two

tasks spread demands over separate subsystems, there is little interference

and time-sharing is efficient.

Insert Figure 1 about here

Mental workload is specified as a vector in the multidimensional space

presented in Figure 1. Workload is defined in terms of resource.demand of a

given task and competition between dual tasks on each dimension. Two types

of performance-based measures can identify vectors in such a space. They

are primary task performance and secondary task performance (Wickens', 1984)-.

Primary task performance is considered a vector measure because it

directly reflects the resource demands imposed by task performance on the

three dichotomous dimensions. However, there is one drawback in this

measurement technique. It cannot reflect resource consumption when a

process 1s data-limited (Norman & Bobrow, 1975). When a process becomes

data-limited, performance maintains at an asymptotic level even as subjects

actually invest more resources to do the task.

The resource demand of a task may also be measured by the secondary

task technique. A secondary task is added to absorb the residual- resources

left from a primary task. Therefore, secondary task performance is

inversely related to the demand of the primary task. The secondary task

technique may also be used to identify a structure-specific vector, imposed

by a primary task on the resource dimensions, by analyzing the interference

patterns between the primary task and a set of secondary tasks. Because
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secondary task performance may be used to determine the locus of resource

demand of a primary task, it is considered a diagnostic vector measure.

Subjective Workload

Although workload may be indicated by the two types of performance

measures, many researchers argue that the subjective aspect is crucial to

the concept of mental workload. Sheridan (1980) stated that subjective

perceptions of cognitive effort constitute the essence of workload. Others

suggested that "if an operator feels effortful and loaded, he is effortful

and loaded" (Johannsen et al., 1979, p. 105). Many researchers take the

view that if you want to measure an operator's workload, you should simply

ask him what his workload is. In addition to this face validity, subjective

measures are considered nonintrusive, cheap, and convenient to be

implemented in any environment.

With these advantages, subjective measures are regarded as: (1)

central to any investigation of workload (Johannsen et al., 1979), (2)

valuable indices of workload (Moray, 1979; Wierwille & Williges, 1978), and

(3) the most sensitive and reliable measures (Gartner & Murphy, 1976).

Furthermore, subjective measures and performance generally show a fairly

high level of correlation across a wide range of tasks and task

configurations. Tasks that are performed more poorly are generally

described as more difficult.

Regardless of their popularity, little effort has been made to

understand the origins of subjective feelings of load (Moray, 1982) and the

limitations of the techniques. Many important questions about subjective

measures remain unanswered. For example, what do subjective ratings
measure? How sensitive are they to the demands on different resource

dimensions? Under what conditions do they dissociate from performance?

In the present study, these aspects are examined. The structure of

perceived workload is revealed-and.is compared with the structure of

processing resources provided by the multiple resources model. Since

performance and subjective workload assess the same hypothetical construct,

a strong correspondence is expected. A model of verbal report data is used

to understand the limitation of subjective introspection. A theory of

dissociation is proposed to specify the conditions in which performance and

subjective workload dissociate. Finally, results from three experiments are

presented to test the hypotheses drawn from the proposed theory. It is

believed that the results of the present study not only advance our

understanding of subjective workload, but-also indicate why performance and

subjective workload dissociate.

The structure of perceived workload: A Multidimensional Scaling

approach. The structure of perceived mental workload has been investigated

by a Multidimensional Scaling (MDS) approach (Derrick, 1981; Derrick &

Wickens, 1984; Yeh & Mickens, 1984). In this MDS approach, tasks were

judged by the similarity of difficulty in order to disclose the hidden

cognitive structure. Results from these studies demonstrated that the

subjective aspect of workload may be .understood by the dimensional structure

that has been empirically verified and provided by the multiple resources

model. The structure of subjective workload was found to be closely related

to the structure of processing resources. The structure was composed either

of three dimensions (input modality, resource competition, and time-demand
in Derrick & Wickens's study), or two dimensions (resource demand and

processing codes in Wickens & Yen's study).

Although the structure of subjective difficulty corresponds with the

structure of information processing and performance, subjective measures are

limited by their own nature. On the one hand, subjective ratings are scalar

measures and hence do not completely preserve the characteristics of a

vector. On the other hand, subjective ratings of workload are verbal report

data and this imposes a potential limitation on their accuracy.

A model of verbal report data. According to Ericsson and Simon's model

of verbal report data (1980), introspection reflects information heeded in

working memory. Any information to be verbally reported has to be in

working memory or has .been transferred to 'long-term memory. •• Automatic' •"' '•'

processes that do not utilize capacity in working memory will not be

available to introspection. Processes whose demands exceed the maximum

capacity of the memory will not be accurately reported because there is less

variation in resource mobilization under these conditions.

This model of verbal report data is consistent with previous findings

in workload measurement studies. Most variables that have been found to

affect subjective workload are related to the demands on working memory.

These variables include memory load and presentation rate (Hauser,

Childress, & Hart, 1982; Daryanian, 1980), the number of tasks-to-beprocessed per processing unit (Tulga & Sheridan, 1980), the requirement of

generating lead and making precise control, rate of information processing,

fraction of attention allocated to the task, insufficient data, number of

decision alternatives, etc. (see Moray, 1982 for a review). Moreover,

Eggemeier, Crabtree, and Reid (1982) showed that subjective ratings were not
as sensitive as was performance in a high memory load condition, as

suggested by Ericsson and Simon (1980).

Gopher and Braune (1983) presented a similar view and suggested that

subjective measures reflect the perceived magnitude of resource investment

in the conscious attention. They argued that we are only aware of part of

the information processing that we do. Thus, subjective estimates follow

the pattern of the most restricted model of a single undifferentiated pool

of resources while performance follows the pattern of a multiple resources

model.

As a consequence of this restricted source of input to subjective

measures, their correlation with performance should not be expected to be

unity or even fairly high across all the conditions. Human performance is

determined by the interaction-of the capacities-of--a large number different

subsystems and the demands imposed on those subsystems. Wickens and Yen

(1983) pointed out that there is no reason why the demands on different

subsystems must be equally read when an operator generates an introspective

rating of mental workload. Dissociation arises when certain subsystems

contribute very heavily to subjective workload estimates but only marginally

to performance. Tasks that impose on those subsystems will be performed

considerably better than their subjective ratings will indicate.

Dissociation also occurs when subsystems contribute heavily to performance,

but are not read by subjective measures. In this case, subjective measures

will provide an overly optimistic view of the expected level of system

performance.
8

A theory of dissociation between performance and subjective workload

Based upon the model of subjective introspection and an examination of

the sources of dissociation that have been found in the literature, Wickens

and Yen (1983) proposed a theory of dissociation from the framework of the

multiple resources model. Subjective workload was proposed to be sensitive

to the amount of aggregate resource investment. Furthermore, if subjective

introspection reflects information heeded in working memory, primarily

represented by the perceptual/cognitive resources, then the demands on these

resources may contribute more to subjective measures than to performance.

Several sources gf increased information processing demands may produce

dissociations between performance and subjective measures. These sources

are listed in Table 1.

Insert Table 1 about here

Things that can be done to a task to increase the workload (decrease

performance and/or increase the subjective feeling of effort) are listed as

the set of manipulations in the first column. Within the second column are

numbers that indicate the extent to which each manipulation will deteriorate

performance (P). The numbers within the third column indicate the increase

of subjective difficulty (S) produced by each manipulation. The particular

values of these numbers are somewhat arbitrary, but what is important is the

relative value of these two numbers, P and S, for a given source. In other

words, the numbers are ordinal and only the relative order between the

numbers is meaningful. The ratio or the distance of two numbers provides a

qualitative description rather than an absolute quantitative value.
Table 1: A theory of dissociation

Sources P. (decrease) 1 (increase)

1. Increased single task difficulty 4 3

Perceptual/Cognitive 2 2

Response 2 1

2. Concurrent task demand . • 3 -.4

Same resources 2 2

Different resources . 1 2

3. Motivation -1 +1

- Resource investment
This theory proposes that manipulating the parameters of a single task

generally influences P as much or more than S. This difference is

particularly pronounced if the single task is degraded by imposing demands

on responding, rather than on perceptual/cognitive processing. The theory

also suggests that increasing workload by increasing the number of

concurrent tasks generally serves to increase S and decrease P, but the

former by a greater degree than the latter. In addition, subjective

experience of workload is postulated to be uninfluenced by whether those

tasks compete for common or separate processing resources. But, performance

suffers substantial losses when common resources are employed in both tasks

and is little affected when demands are spread over separate resources. A

final source of dissociation is related to any variable that induces more

resource investment to improve task performance. Subjective workload is •

predicted to be increased by this variable even if the investment helps

performance to a great extent.

This theory has been partially supported by previous studies and in

fact was formulated in part to account for the data of those studies. In

Derrick's study (Derrick, 1981; Wickens & Derrick, 1981; Derrick & Wickens,

1984), the difficulty of a manual control task was varied by three means:

(1) increasing the control order of the task which demands both

perceptual/central and response resources, (2) adding a concurrent task (an

auditory memory search task or a tone judgment task) which demands nonoverlapping resources, and (3) pairing the task with itself or with a visual

search task which competes for common resources. All of these manipulations

increased demands upon the processing system and thus induced decrements in

performance and increments in subjective ratings of workload. Dissociation

was found between the two measures under these manipulations. Subjective
10

workload was higher under an easy dual task condition than under a hard

single task condition^ even though performance was better under the former

condition.

In a study conducted by Wirkens and Yeh (1983), the difficult y of a

tracking task was manipulated by three means: (1) the control order or the

bandwidth of a first-order tracking task was increased, (2) when performing

a second-order tracking task, subjects tracked wit h or without a predictor,

and (3) the tracking task was paired with itself or with a Sternberg memory

search task whose input and output modalities were manipulated to vary the

degree of resource competition between two tasks.

Replicating Derrick and Wickens' finding , subjective workload was

higher but performance was better when doing two easy tasks than when doing

a hard single task (Figure 2a). This result supports the theory of

dissociation and the model of verbal report data suggested by Ericsson and

Simon (1980). Even if there are separate resources availabl e to perform two

tasks, the perceptual/central resources are still highl y demanded as an

executive to coordinate the processing and the execution of both tasks.

Relative to the demand of a hard singl e task, this "cost of concurrence"

(Navon & Gopher, 1979) enhances the demand for perceptual/cognitive

resources to a greater extent.

Insert Figur e 2 about here

Wickens and Yeh (1983) also found a dissociation when different singl e

task conditions were compared. (1) For an equal level of subjective

workload, performance decrements were larger when performing a highbandwidth task than when performing a second-order tracking task (Figure
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Figure 2. Dissociations found in a previous studv
11

2a). Limited by a small sample of subjects, Wickens and Derrick (1980)

tentatively suggested that a high bandwidth tracking task demanded stagenon-specific resources but a second-order tracking task demanded both

perceptual/cognitive and response resources. Therefore, a second-order

tracking task may demand more perceptual/cognitive resources than does a

high-bandwidth tracking task. As a consequence of these different demands

on perceptual/central resources, subjective workload overestimated the

performance decrements in a second-order tracking task. (2) A predictor

display in a second-order tracking task aided performance by providing a

precise control information. Nevertheless, in order to utiliz e the

information, subjects had to invest more resources to process two task

elements (i.e., cursor and predictor). Thus, subjective workload under a

predictor.display condition was higher-tha n what its performance woul d .

indicate (Figure 2b).

In Wickens and Yeh's study (1983), dissociation was also found when

different dual task conditions were compared. For an equal performance

level, subjective workload was higher in a low resource competition

condition (tracking and an auditory-speech memory task) than in a hig h

competition condition (two tracking tasks) (Figur e 2a). The insensitivity

of subjective measures to resource competition, once again indicated that

some information was not read equally by the two measures. The executive

management was equally engaged in coordinating two tasks and the aggregate

resource investment was simila r in both hig h and low resource competition

conditions. Since the aggregate investment contributed more to subjective

measures than to performance, subjective workload was not as sensitive to

the degree of resource competition as was performance.
12

Experimental Overview

Although the theory of dissociation proposed by Mickens and Yeh (1983)

has been partially supported, a systematic approach was adopted in the

present study. Converging evidence can provide a thorough test of the

theory and can advance our understanding of subjective workload. Sources of

information processing that may produce dissociation were directly

manipulated and hypotheses drawn from the theory were tested. Three tasks:

a Sternberg memory search task, a one-dimensional compensatory tracking, and

a dual-axis tracking task were used in three experiments. In the Sternberg

memory search task, demands were imposed upon different stages of processing

or different codes of central processing. In the one-dimensional

compensatory tracking task, the'b'andw'idth or the .control order was-:-vari'ed tocreate the difficult conditions. Display augmentation was implemented to

aid performance in two higher order tracking task conditions. In the dualaxis tracking task, the effect of separating display or control was

investigated. In dual task conditions, the degree of resource competition

was manipulated via various combinations of the tracking tasks and different

Sternberg memory search tasks. The patterns of dual task interference were

analyzed by the additive factors methodology in order to make inferences

concerning the resource demands of various tracking tasks. The following

section provides a rationale for the specific manipulations'that were chosen

for testing each hypothesis.
13

Hypotheses of the dissociation

Hypothesis 1 : Subjective ratings are dominated by the demands on

perceptual/central resources. -According to the theory, dissociation will

occur when two manipulations increase their demands on different processing

stages. In the first experiment, demands were placed on a specific

processing stage (i.e., perceptual load, central load, or response load) to

increase the difficulty of a Sternberg memory search task. Each difficulty

manipulation was assumed to effect the amount of demands on one stage

without altering the demands on other stages. If the hypothesis is correct,

then dissociation should be found when the perceptual or central load memory

task was compared with the response load condition. For an equal

performance level, subjective workload should -be greater under-the

perceptual and central load conditions than under the response load

condition.

The integrality of a cursor in a dual-axis tracking task was

manipulated in the third experiment. In one condition, two cursors were

tracked by one joystick to impose extra demands upon perceptual/central

resources. In another condition, one cursor was controlled by two joysticks

to increase the demands on response resources. A dissociation was predicted

when these two dual-axis tracking tasks were compared.

Hypothesis 2: Subjective ratings are driven more by the number of

concurrent tasks whereas performance is relatively more effected by the

difficulty of single tasks. This hypothesis was postulated to replicate

previous findings in different task configurations. In the first

experiment, the difficulty of a single Sternberg memory search task was
14

increased by enhancing the demands on specific processing stages. The

difficulty of a tracking task was manipulated by increasing the bandwidth.

The difficulty was also increased by combining the two easy tasks in a dual

task condition. In the second experiment, the difficulty of a tracking task

was raised by increasing the bandwidth, the control order, or by adding a

basic memory search task in a dual task condition. In the third experiment,

the difficulty of a dual-axis tracking task was varied through the demands

on perceptual or response resources in single task conditions. The

difficulty was also increased by combining the tracking task with an easy

memory search task in a dual task condition.

The number of display elements affects subjective workload as much or

relatively more than it.influences.performance. A related test of-the first

two hypotheses is that the number of display elements drives subjective

workload more than it effects performance. Murphy, McGee, Palmer, Paulk,

and Wempe (1978) showed that when information presented in the center of a

flight control display was cluttered with a number of motion indicators,

performance was better but subjective workload was higher in this condition

than in the other two display conditions in which fewer motion elements were

visible. Garner (1974) showed that integral and separable dimensions

influence subjective perception. Perceptual features are perceived as one

unit when they interact integrally and as distinct units when they are

separable. Kahneman and Triesman (1984) proposed a similar concept (i.e.,

the notion of "object file"). Features of an object are perceived as a

whole and attention is directed by the object rather than by the composite

features. When there is more than one object, attention has to be divided
15

between objects. Kramer (1984) also showed that the number of object units

effected dual task performance.

Wickens and Yeh (1983) found that an external predictor in a secondorder tracking display improved performance but did not decrease subjective

workload. In interpreting the result, they suggested that the external

predictor was separate from the tracking cursor and was hence perceived as

another unit in working memory. Resource demands on working memory under

the predictor display condition were higher than the demands of a tracking

task in which the external predictor was absent. Therefore, subjective

workload was higher under the former condition than what its performance

would indicate.

The integrality of a predictor with a tracking cursor .was manipulated

in the second experiment." In one second-order tracking condition, the

predictor was external to the cursor. It was assumed that processing two

distinct elements, in comparison wit h a display without the predictor, would

demand extra perceptual/central resources, but that the predictor symbol

would nevertheless improve performance. In another condition, the

predictive information was integrated wit h the cursor so that only one

object unit was processed. If the number of object units affects the

perceived demands in addition to the objective demand, a dissociation wil l

occur. For an equal performance level, integrated features wil l be

perceived as less demanding than separate features.

Hypothesis 3 : Subjective ratings are not as sensitive as is.

performance to resource competition. If subjective ratings measure the

aggregate demands, ratings should be roughly equivalent in both high and low

resource competition conditions but performance wil l depend upon the amount
16

of resource competition. In previous studies (Derrick & Wickens, 1984;

Wickens & Yen, 1983), this dissociation was shown when the difference in the

amount of competition was large between two dual task conditions (dual

tracking tasks vs. tracking with an auditory memory task).

In the present study, the amount of resource competition between two

tasks was restricted to certain resources. In Experiment 1, the degree of

competition for specific stage-related resources was varied in different

dual task conditions. A tracking task presumed to be "response loading",

was performed concurrently with a response load memory task (high

competition) or with a perceptual/central load memory task (low

competition). The competition for the codes of central processing was

manipulated in the second and third experiment. A tracking task was

combined with-a spatial memory, task (high resource competition) or.-with a" ;

verbal memory search task (low competition). It was predicted that

dissociation would occur when the low and high resource competition

conditions were compared. Performance decrements would be larger in the

high competition conditions but subjective workload would be relatively less

sensitive to the difference in the amount of resource competition.

Hypothesis 4 : Factors which induce more resource investment to improve

performance will also increase subjective workload. Moray (1982) suggested

that the degree of precision required by a task and the probability of

failure affect subjective mental load. The degree of precision in a highbandwidth tracking task was used in the second experiment to motivate

subjects to invest more resources to improve performance. It was predicted

by the theory that this investment would also increase subjective ratings of

workload and dissociation would occur.
1.7

Tulga (1978) argued that subjects may lower the performance criterion

in a high load condition when they perceive a large discrepancy between the

desired and the actual performance. When subjects lower their criterion,

their performance will drop off but they will feel less loaded. To test

this potential source of dissociation, two degrees of precision were imposed

upon subjects in different high-bandwidth tracking task conditions in the

second experiment. A very precise control was required in one condition so

that the subject's actual performance would be far from the objective

demand. In another condition, the precision was eased so that the perceived

discrepancy between the actual and the objective performance would be

relatively smaller. If subjects lower their performance criterion in the

former condition, dissociation between the two workload measures, will occur.

Secondary issues

In addition to verifying the theory of dissociation, several secondary

issues were also investigated in the present study. These issues include:

The resource demands of a high-bandwidth tracking task and of a dualaxis tracking task with separated display/control. Wickens and Derrick

(1980) tentatively suggested that the bandwidth manipulation requires stagenon-specific resources. Manipulations similar to those used in the

preceding experiment were employed to verify this suggestion. In the first

experiment, the bandwidth manipulation was combined with the manipulation of

Sternberg variables on specific processing stages in various dual task

conditions. The locus of resource demand of the bandwidth manipulation was

analyzed by the additive factors method. The bandwidth manipulation was

combined with the manipulation of processing codes in the second experiment
18

to test whether increasing bandwidth places demands on a specific code of

central processing. The same method was also applied to investigate the

locus of resource demand of a dual-axis tracking task with display or

control separated.

The reliability of subjective ratings. Although subjective measures

have been widely used, the reliability of these ratings has seldom been

investigated. Yeh and Wickens (1984) showed that post trial ratings were

less reliable than post session ratings even though both were fairly

reliable. In the present study, this issue was examined to replicate such a

difference in rating reliability because the implication of this issue is

important. If operators are more consistent in rating when they have a

chance to compare the demands of all the systems, ratings should be

collected in the context of all of the just-performed systems. Ratings

collected immediately after operators test a system may be less reliable.

Predicting a global workload scale from specific scales. Using a

multiple regression analysis, the rating on a global workload scale may be

treated as the dependent variable and the ratings on other scales may be

treated as the predictors. The multiple R square indicates the amount of

variance in the global workload rating that is accounted for by the

predictors. The regression weight of each predictor represents-its •' •

importance in predicting the global rating. If the weight is not

significant, then this predictor is not a salient component of the global

workload measure. Vidulich and Wickens (1984) found that only three

specific scales contributed to explain the variance in the ratings of two

global scales. These scales are mental/sensory effort, response load, and
19

stress level. Two of the scales are directly related to the resource

dimensions described by the multiple resources model.

The perceived structure of subjective workload. Results from previous

studies (Derrick & Wickens, 1984; Yeh & Wickens, 1984) demonstrated that

there is a strong relation between the perceived structure of subjective

difficulty and the structure of processing resources. The multidimensional

scaling approach was again adopted in the present study to disclose the

hidden cognitive structure which may not be in the conscious awareness of

the subjects. This evidence may provide a complete picture of the structure

of subjective workload. Furthermore, the nature of subjective workload

revealed from the MDS analysis may be combined with the results of the

regression analysis to-help the understanding of the performance-subjective

workload dissociation.

Methodology of the analysis

An analysis method is necessary to test whether different manipulations

have differential effects on the performance and subjective ratings. The

interaction effect of an ANOVA provides such an information. Prior to the

analysis, performance and subjective workload measures must be scaled into

common units in order to test the relative effects of two manipulations.

However, performance and subjective measures differ in their nature.

Subjective ratings are scalar measures whereas performance is itself a

vector quantity, sometimes reflecting one, and sometimes two tasks, and

having speed and accuracy components. Performance from each facet must be

integrated into a scalar measure to reflect the total effect of a
20

manipulation. In addition to providing the dependent variable for the test

of dissociation, this integrated performance measure may be used to examine

the interference pattern between two tasks. The interference pattern is

used to infer the processing demands of various tracking tasks via the

additive factors methodology. Each of these points is discussed in the

following section.

Test of dissociation

Assuming that the two measures are expressed in comparable units, a

dissociation may be characterized in a space with subjective ratings of

workload and performance measures as the two axes. Ideally, if a

manipulation drives the two measures to the same degree, its effect as

measured from a baseline condition will result in a 45 degree vector in the

space. Both the ratio of ".change in performance to change, in-subjective '-*•

measures (AP/AS ) and the correlation between the two measures will be 1.0

(Vector a in Figure 3a). When a manipulation effects one measure relatively

more than the other, the vector will deviate toward one of the axes (Vector

b in Figure 3a).

Insert Figure 3 about here

Increases in demands from a baseline condition (e.g., a first-order low

bandwidth tracking task or a Sternberg task with memory set of one) may be

imposed by different means. When different manipulations drive the two

measures to the same degree, the resulting two vectors will have the same

slope (Vectors b and d in Figure 3a). When manipulations drive the two

measures to different degrees, the resulting vectors will diverge (Vectors b

and c in Figure 3b). A significant divergence between two vectors indicates
Poor

Easy Difficult

Subjective workload

Poor

Good

Easv Difficult'

Subjective workload

1 2

Manipulation.

d.

S2

1 2

Manipulation

Ficure 3. A hvcothetical dissociation and the test of dissociation
21

a dissociation between the two measures as the consequence of different

manipulations.

With the baseline condition(s) represented as the origin of the

vectors, a dissociation is defined by two characteristics. (1) For an equal

level of subjective workload (Point A in Figure 3a), there is a significan t

difference between the performance measures (Point B vs. C in Figur e 3a).

(2) For an equal performance level, there is a significant difference

between subjective ratings of workload (Figure 3b). If there is no

significan t dissociation between the two measures, then PI/SI wil l be the

same as P2/S2 and there will be no interaction between the measure type and

the manipulatio n type (Figur e 3c). If there is a significan t dissociation,

then the measure type wil l significantl y interact with the manipulatio n type

(Figur e 3d). Therefore, a significan t interaction in the ANOVA, with the

manipulatio n type and the measure type as two factors, is used to indicate

diverging vectors in the performance-subjective ratings space.

Workload measures in comparable units

How should different types of measures be scaled in comparable units in

order to test a dissociation? Subjective measures are usually assessed by

certain rating values (i.e., interval or ratio scales). On the other hand,

performance measures are recorded in various forms such as latency,

accuracy, or tracking error. There are three transformation methods which

may be employed to produce comparable units.

(A) Standardized scores. In this method, a grand mean and a standard

deviation are obtained from all the subjects and all the conditions for each

dependent measure. Each subject's score is then standardized by the grand

mean and the standard deviation. This procedure is executed for all facets
22

of performance and for subjective measures. Various performance variables

are then weighted and combined to produce one performance measure in each

dual task condition. After this transformation, both performance and

subjective measures are on the -standard units.

(B) Estimates of effect magnitude. For each dependent measure, an

effect size is derived for each difficult condition (see Hedges, 1981, 1982;

Ackerman, in press for a complete discussion). Basically, the effect size

is the t-value obtained by (1) subtracting the group mean of the control

condition (easy single task) from the group mean of the experimental

condition (e.g., a dual task condition or a hard single task), and (2).

dividing this difference by the weighted variance of the two conditions.

The effect sizes (t-values) of the dependent measures are then combined to

estimate the overall magnitude* of effect in a dual task condition .or a hard

single task condition. The subjective measures and performance are now on a

common metric of effect magnitude.

(C) Decrement scores. To obtain the effect of one manipulation on a

control task, a difference score is first derived for each dependent

variable and for each subject. For each measure, the variance in the

difference scores is computed across all the subjects for each condition and

the mean variance across all the conditions is used as the denominator to

normalize the difference scores. This method is a modified version of the

one used by Wickens, Mountford, and Schreiner (1981). The normalized

decrement score combined between two tasks is then used as the performance

measure of a dual task condition.

In essence, both of the latter methods attempt to measure the size of

an effect relative to the control conditions. However, the decrement

measure (Method c) differs from the method of estimating effect size in the
23

derivation of variances. In Method c, Individual decrement scores rather

than the raw scores are used to obtain the variance of each condition.

These individual decrement scores are used so that the effect size of each

subject's performance or the effect size of the rating is not influenced by

other subjects' effect sizes. These measures are particularly appropriate

for subjective ratings because different subjects use the rating scales in

various ways. Subjects vary widely in the values and the ranges given to

tasks. In addition, the variance is averaged across all the conditions in

Method c, instead of the two conditions that are of interest in a

comparison. The basic assumption underlying this averaging is that the

decrements in all of the conditions are drawn from one distribution.

Test of-the resource.demand of .a task: The additive' factors •metho'dolo'gy'

The additive factors method was originally employed to analyze the

latency of a mental process into distinct stage components (Sternberg,

1966). One manipulation factor is assumed to prolong the duration of a

particular stage without altering the latency of any other stage. Since

stage durations are additive by definition, the changes in mean RT produced

by two factors will be independent and additive if they effect different

stages. When two factors influence the same stage, then effects of the two

on RT will be interactive. This method has been applied in dual task

researches to localize the- processing resources that overlap between a

manipulated Sternberg task and a primary task (Wickens, 1978; Micalizzi &

Wickens, 1980). When the Sternberg manipulation and the primary task

manipulation consume the same resources, the increase in latency of both

manipulations together will be greater than the sum of the independent
24

increases. When the two demand non-overlapping resources, additivity

occurs.

In the present study, the dependent variables characterizing the

performance of a single memory-task were the average of the latency and the

accuracy under that condition. The dependent variable of a dual task

condition was a score combined from the performance of each component task

(i.e., RMS, RT, and accuracy). It was assumed that the effect of a

manipulation on resource dimensions may be manifest in all the facets of

performance. Therefore, the single latency dependent variable may not truly

reflect the effect of a manipulation because subjects may adopt a tradeoff

strategy. For example, subjects may trade accuracy for a faster reaction

time in single memory task conditions, but not in dual task conditions. Or,

they may protect memory task performance in a dual task condition by trading

the tracking performance. Thus, it was assumed that the combined

performance measure reflects the total cost imposed by the concurrent

resource competition, independent of resource allocation between two tasks.

Therefore in the present study, after being transformed to comparable

units, performance measures from all facets of processing are integrated.

The integrated performance measure represents the total effect of a

manipulation on the human information processing system. This integrated

measure, employed with the additive factor methodology, is used to test for

the interference patterns of various dual task conditions. The results are

used to infer the demand of (a) a high-bandwidth tracking task, (b) a

second-order tracking task with a predictor, and (c) a dual-axis tracking

with display or control separated. These various tracking tasks were

combined with different Sternberg-task manipulations in dual task conditions

across the three experiments. When a Sternberg-task variable and a tracking
25

difficulty manipulation demand the same resources, the effects of the two on

the integrated performance measure are assumed to be interactive. When the

demands of the two manipulations are spread over separate resources, the

effects are assumed to be additive.

In the following section, the results of three experiments are

presented in concert because the data are related to the same issues and

test common hypotheses.
26

Experiment 1

Various combinations of a tracking task and a Sternberg memory search

task were employed to investigate three hypotheses. To review, these

hypotheses state that (1) Demands upon the perceptual/central resources

drive subjective ratings more than they drive performance. On the other

hand, response complexity drives performance more than it drives subjective

ratings of workload. (2) Performance measures rather than subjective

ratings are sensitive to resource competition. For an equal level of

performance, subjects feel more loaded when performing a low competition

dual-task combination than when performing a high competition condition.

(3) Dissociation occurs when a factor induces more resource investment to

improve performance. : • • - " .•.-' . '.;-• .

The difficulty of a single memory search task was increased by imposing

more demands upon a specific processing stage. The manipulations resulted

into three hard single task conditions (i.e., perceptual, central, and

response load memory tasks). Performance and subjective measures of these

three conditions were then contrasted to test the first hypothesis. The

bandwidth of a tracking task was increased in the difficult condition.

Manipulations of Sternberg variables and of tracking bandwidth were combined

in dual task conditions to (1) infer the resource demand of a high-bandwidth

tracking-task, and to (2) test the second hypothesis on the effect of

resource competition. To test the third hypothesis, the degree of precision

in a high-bandwidth tracking task was manipulated as a factor that motivates

subjects to invest more resources.
27

Method

Subjects

. Fifteen students of the University of Illinois participated in this

experiment. All subjects were right-handed, native speakers of English, and

had normal vision.

Apparatus

Subjects were seated in a light and sound attenuated chamber. The

tasks were implemented on a PDP-11/40 computer. The computer was interfaced

to a 10 cm x 8 cm CRT display via a Hewlett-Packard 1300 Graphics Display

Interface. The display was about 90 cm in front of the subject and slightly

below eye level. The subject's responses for a memory search task-were"-.. ':

accomplished through a push button control panel affixed to the right

armrest of the subject's chair. The subject's input for a manual control

tracking task was via a MSI 521 joystick affixed to the left armrest of the

subject's chair.

Tasks

Sternberg memory search task. Prior to each trial of the basic memory

search task (BM), the subject viewed one alpha-numeric string (e.g., A26).

The string was held in working memory for the next two minutes as the.

subject was presented a series of probes, half of which were in the memory

set. Subjects were asked to press the first key on the panel if a probe.was

in the memory set (i.e., a "go/no-go" response). The demand on each

processing stage was increased via different manipulations (Derrick &

Wickens, 1980). In the perceptual load condition (PM), a grid mask was
28

superimposed upon each probe stimulus. In the central load condition (CM),

memory set size was increased to three (e.g., A26, C59, R89). In the

response load condition (RM), subjects had to make double responses for

every positive response and for every negative response. Subjects had to

press key 2 then key 4 for a positive response. They had to press key 3

then key 5 for a negative response (Figure 4a). Subjects had to release the

first key completely before they pressed the second key and they had to

press the second key within a window of 600 msec. Responses outside this

window were recorded as non-responses. Reaction time of the first response

was used as the latency of the response load condition. The subject's task

in all of these conditions was to respond as accurately and as quickly as

possible.

Insert Figure 4 about here

Tracking task. In the easy condition, subjects were required to

nullify an error cursor that was displaced horizontally by a random forcing

function with a cutoff frequency of 0.32 Hz. The error cursor was

controlled by the left-right movement of the joystick with the left hand.

Only the first order system dynamics were utilized. The bandwidth was

increased to 0.54 Hz in the difficult condition.

When subjects tracked the cursor in a "high-bandwidth with a window"

condition (Figure 4b), an acceptable error range was presented. This

manipulation was employed to motivate subjects to invest more resources to

improve performance. Subjects were told that this window indicated a safe

altitude level and that they should keep the cursor within the window as

much as possible 1n order to keep the control of the plane. An auditory
A29 D

D

High BW tracking

Wide window

Narrow window

a. Response pattern in the

resoonse load condition

b. High-bandwidth tracking

DNo augmentation

n

Predictor display

c. An example of a sptial

memorv set

Command display

d.- Second-order tracking

'igure 4. Response pattern in the response load condition

and some displays seen by the subjects
29

tone was also given when the cursor was outside the range until subjects

tracked the cursor back into the window. The width of the window was also

varied (i.e, narrow and wide window conditions) to investigate the effect of

the perceived discrepancy between the desired and the actual performance.

Dual' task. Dual tasks were generated by combining one of the four

Sternberg memory search tasks (i.e., basic, perceptual load, central load,

and response load) with a low-bandwidth or the "no-window" high-bandwidth

tracking task. Subjects were instructed to give equal priority to each

task.

Procedure

Practice sessions. Subjects practiced all of the tasks extensively in

the first two sessio.ns over a period of .two days. A.histogram of RMS error

"data was obtained from each low-bandwidth tracking task condition. These

data were averaged for each subject to represent his average performance

under the easy tracking condition and were used to compute the window width

employed in the experimental sessions. The width was chosen so that 15% of

an individual subject's tracking error distribution would fall in the

"narrow" window and 30% would fall in the "wide" window range.

Experimental session, Session 3. Prior to this session, subjects were

given a list of all the rating scales (the descriptions are listed in Table

2). The experimenter went through the list with the subjects and made sure

that they understood the meaning of the scale and the way to do the rating.

Subjects performed the basic memory search task first and they were

instructed to use the subjective experience of doing this task as the

standard which was arbitrarily assigned a rating of 100. Subjects then

performed the other tasks and rated each one against the standard on eight
30

rating scales. They were told to assign a rating of 200 to a just-performed

task if the subjective experience was twice the experience of doing the

standard task and to assign 50 if it was half. This strategy was not

adopted for the "excess capacity" scale on which subjects were allowed to

assign any number between 0 and 100 for every task.

Insert Table 2 about here

The unidimensional ratings were collected at the end of each trial

immediately after subjects performed the task and at the end of the session.

Subjects were then given 20 pairs, randomly chosen from possible paired

combinations of 14 tasks (the two window conditions were not included), and

were asked to. rate, each-pair on a. 9-point. similarity scale. (Table 2-).. •• •

Experimental session, Session 4. Subjects followed the same procedure

as in Session 3 except that all the 91 possible paired combinations of the

14 tasks were rated on the 9-point similarity scale. Unidimensional ratings

collected in this session were compared with the corresponding ratings

recorded in Session 3 to estimate the reliability of the ratings. The

similarity judgments of the 20 pairs collected in Session 3 were collated

with the corresponding ratings in this session to compute the reliability of

the similarity ratings.

Experiment 2

The purpose of this experiment was to verify two hypotheses: (1)

subjective workload is affected by perceptual features, and (2) subjective
Table 2 - Description of the scales used in Experiment 1

Scale Description

Overall workload- (WK) - The total experience during the task, including

how much attention that you paid and how

difficult the task was.

Perceptual effort (PE) - How much effort did you make in perceiving the

stimuli presented to you in the last task?

(e.g., seeing, looking, scanning etc.)

(ME) - How much effort did you make in the mental

; •• ;-. activity, (e.g,-monitoring, thinking, deciding,

planning, remembering, understanding etc.)

(RE) - How much effort did you make in responding

during the last task?

(TI) - How much time pressure did you feel in doing

the last task? How busy were you?

Mental effort

Response effort

Time pressure
89

Table 2 (continued) - Description of the scales used in Experiment 1

Scale Description

Stress (ST) -

Performance

Overall effort

Excess capacity

(PR) -

(OE) -

(EX) -

How anxious/worried/uptight/harassed or

calm/tranquil/placid 'did you feel durin g the

last task?

How successful was your performance?

What was the overall effort that you put in

the last task?

Assumin g that all the effort capacity you have

. is .100. How much was left in doing the last

task. You may use any number between 0 and 100.

The rating scale for judgin g the similarit y of task difficult y

12345. 6 78 9

Very Similar Inter- Dissimila r Very

Simila r mediate Dissimila r
31

measures are less sensitive to the competition between the codes of central

processing.

In order to test the first hypothesis, the integrality of a predictor

with the cursor was manipulated. It was assumed that integral features

would be perceived as a unit whereas separable features would be processed

as distinct units. If the number of object units affected subjective

workload, subjects would feel less loaded when perceiving integral features

than when processing separable features even if performance measures were

roughly equivalent.

To test the second hypothesis, the degree of resource competition was

manipulated in dual task conditions. It was presumed that resource

competition would be higher when a tracking task was performed with a

spatial Sternberg memory search, task than when it was time-shared with a

verbal Sternberg memory search task. Performance was predicted to be

sensitive to the difference in the amount of competition, but subjective

workload would be affected by the aggregate demand.

Method

Subjects

Fifteen students of the University of Illinois were subjects in this

experiment. All subjects, were right-handed, native speakers of English, and

had normal vision.

Tasks

Sternberg memory search tasks. In the verbal memory task (BM),

subjects remembered and responded to three sets of alpha-numeric strings as
32

in the central load condition in Experiment 1. In the perceptual memory

task (PM), subjects held two sets of strings in memory and a grid mask was

placed upon every probe. In the spatial memory task (SM), subjects

remembered and responded to two sets of dot patterns. The dot patterns were

chosen from the ones used in Sandry and Wickens 1 study (1982) (see Figur e 4c

for an example). The go/no-go response assigned to the memory tasks in the

first experiment was employed in all of these tasks.

Tracking task. In addition to the low-bandwidth and the "no-window"

high-bandwidth tracking tasks employed in the first experiment, three

additional kinds of tracking tasks were added in this experiment. Figure 4d

illustrates the displays seen by the subjects in these three conditions. In

the unaided second-order tracking task, acceleration dynamics were employed.

In the "predi.ctor: displ-ay"; condition,--a predictor symbol, driven by-the '

estimate of the cursor's current velocity and acceleration, was displayed

above the tracking rectangle. In the "command display" condition, the

principle of the "pseudo-quickening" display (see Gill , Wickens, Donchin, &

Reid, 1982 for the specifications of the method) was adopted. A line

£

segment was added to the cursor to indicate the direction of the required

control input by providing information concerning the higher derivatives of

the system state. The direction of the segment changed when reversal should

be implemented in control direction as specified by the theoretical

switching line. Intuitively, the direction of the segment was the same as

the subject's control movement when an undershoot needed to be corrected.

The direction switched when the system velocity, relative to the system

position, would produce an overshoot if reverse movements were not made

immediately. In all of the three second-order tracking tasks, the gain of

the system was lower than that in the other tracking conditions.
33

Dual task. The low-bandwidth or high-bandwidth tracking task was timeshared with the verbal memory search task or the spatial memory search task.

The perceptual memory task was performed concurrently with each of the three

second-order tracking tasks.

Procedure

Subjects practiced the tasks in the first two sessions. Data were

collected in Sessions 3 and 4. The low-bandwidth tracking task was used as

the standard task to rate other just-performed tasks. Most of the rating

scales were the same as those used in the first experiment with the

following changes: (1) the perceptual effort and mental effort scales were

combined into one scale, (2) the performance scale and the overall effort

scale were not used, in this experiment,- and ..(.3.) a task -complexity scale and

a modified Cooper/Harper scale were added (see Table 3 for the description

of these new scales). The task complexity scale was chosen from one of the

categories proposed by Sheridan and Simpson (1979). The modified

Cooper/Harper scale was added because the original scale has been used in

5

many studies and Wierwille and Casali (1983) have validated and recommended

this modified version for assessing overall workload. The ratings collected

in the two experimental sessions were used to compute the rating

reliability.

Insert Table 3 about here

At the end of Session 3, fifteen pairs drawn from the combinations of

11 tasks (the four conditions that involved the perceptual load memory task

were not included) were rated on the similarity scale. At the end of
Table 3 - Description of the scales used in Experiments 2 and 3.

Scale Description

Overall workload (WK)

.Perceptual/Mental effort (P/ME)

Response effort.

Time pressure

Stress

Task complexity

Capacity demand

(RE)

(TI)

(ST)

(CM)

(CA)

same as the scale used in Experiment 1

How much effort did you make in

perceiving the stimuli presented to you

and in the mental activity (e.g., seeing,

looking, scanning, monitoring, listening,

thinking, deciding, planning,

understanding, etc.)

same as the .scale, used .in Experimental- ;

same as the scale used in Experiment 1

same as the scale used in Experiment 1

How complex was the task due to

uncertainty, unpredictability,

unfamiliarity, or due to automated versus

skilled planning required?

Assuming that all the effort capacity you

have is 100. How much did you use in

doing the-task? - You may use any number

between 0 and 100.
Table 3 (continued) - Description of the scales used in Experiments 2 and 3.

Scale

Cooper/Harper

Description

- Please rate the task by going through the

following decision tree. It is important

that you go through it step by step.

You should start from the operator

decision. Based on your answer to .that

question, you either go up to ask

yourself the next question or go to the

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34

Session 4, all of the 55 possible paired combinations of the 11 tasks were

rated on the similarity scale. The similarity ratings of the same 15 pairs

in the two sessions were then collated to check the reliability of the

similarity ratings.

Experiment 3

The purpose of this experiment was to generalize the findings from the

first two experiments by employing various combinations of a Sternberg

«

memory search task and a dual-axis tracking task. In the basic dual-axis

tracking task (BT), errors on the X and Y axes were integrated into one

cursor on the display and were controlled by one joystick. The difficulty

of this :task.was manipulated by impo'sing-demands on stage-specific

processing resources. It was assumed that separating the integrated cursor

into two distinct indicators, with each cursor representing the error on one

axis, would increase demands for perceptual/cognitive resources. On the

other hand, controlling an integrated cursor by two separate joysticks would

presumably demand more response resources. The difficulty of the tracking

task was also increased by adding a Sternberg memory search task which

demanded common or separate resources from the tracking task. The pattern

of interference between the Sternberg and the tracking tasks could be used

to check the locus of processing load of the two manipulations.
35

Method

Subjects

Fourteen students of the University of Illinois were subjects in this

experiment. All subjects were right-handed, native speakers of English, and

had normal vision.

Tasks ' •

Dual-axis tracking task. The objective of the basic task (BT) was to

keep a cursor changing in two dimensional coordinates on the stationary

target, a cross in the center, by controlling a single joystick. The

dynamics of the tracking control were first order and the upper cutoff

frequency was 0.30. In the "separated-display" condition (DT), two cursors

signified the errors on each axis respectively and were tracked by one

joystick. In the "separated-control" condition, subjects tracked one

integrated cursor by moving two separate joysticks, one moving the fore-aft

direction and the other in the left-right direction.

Memory search task. In the basic verbal memory task (BM), subjects

remembered and responded to two sets of alpha-numeric strings. In the

spatial memory task (SM), they held-two sets of dot patterns in working

memory and made a positive response when a probe was in the memory set. In

the central load condition (CM), subjects had to remember and respond to

four sets of alpha-numeric strings. In all of these conditions, a manual

go/no-go response was required to minimize the response selection load. In

the speech condition (SP), they remembered two sets of strings and said

"yes" when a probe was in the memory set.
36

Dual tasks. Each of the four memory tasks was combined with each of

the three dual-axis tracking tasks.

Procedure

Subjects practiced all the tasks in the first two sessions. The basic

dual-axes tracking task was used as the standard to rate other tasks in

Sessions 3 and 4. The rating scales were the same as those used in

Experiment 2. Similarity judgment data were also collected in Sessions 3

and 4 to compute the rating reliability.
37

Results

The processing demands of various tracking tasks

Experiment 1. Raw performance scores and subjective ratings of overall

workload from Experiment 1 are presented in Table 4. As shown in this

table, reaction time of the basic Sternberg memory search task was prolonged

as the difficulty was increased. This effect was observed under single task

conditions (imposing demands upon specific processing stages) and under dual

task conditions (low-bandwidth and high-bandwidth tracking with the memory

task). Performing the memory search task with a low-bandwidth tracking task

aggravated the RMS tracking errors. The tracking decrement was magnified

when the memory task was made more difficult* particularly when-the response

load was imposed. RMS errors also increased in the high-bandwidth tracking

task but reduced with the presence of a window. Each type of performance

measure was standardized and combined to create the dependent variable to

which the additive factors method was applied. The combined scores were

then used to determine the locus of resources demanded by the easy tracking

task and by the bandwidth manipulation.

Insert Table 4 about here

The primary purpose of localizing the resource demand is to

substantiate the prior assumption of the degree of resource competition in

different dual task conditions. It was assumed that the competition was

relatively higher when a tracking task was performed concurrently with a

response load memory task than when it was combined with other memory tasks.
Table 4 - Raw performance and subjective workload of each condition

Experiment 1

Condition RMS RT AC Workload

Basic Memory

Perceptual Load (PM)

Central Load

Response Load

Low-bandwidth Tr (L )

Low + Basic Mem (LB)

Low + Perceptual (LP)

Low + Central

Low + Response

High-bandwidth

Narrow window

Wide window

High + Basic Mem (HB)

High + Perceptual(HP)

High + Central

High + Response (HR)

(BM-)

(PM)

(CM)

(RM)

(L )

(LB)

(LP)

(1C)..

(LR)

(HI)

(NA )

(WI)

(HB)

(HP )

(HC)

(HR)

.152

.165

.183

.183-

.242

.259

.225

.220

.272

.293

.293

.365

.398

.453

.600

.574

.488

.514

-. .665

.594

.499

.526

.661

.603

99.063

99.433

95.955

99.216

99.319

99.102

• .94.539

97.788

99.301

98.605

94.715

97.009

.983

1.189

1.762

1.575

1.260

1.617

1.804

J..2.-.314

2.076

1.724

2.490

2.211

2.050

2.286

2.812

2.660
38

Through the additive factors method, this assumption can be tested. If the

assumption is valid, then the tracking task should place greater demands on

the stage of response selection and coordination. Time-sharing should be

less efficient when the tracking task was executed concurrently with a

response load memory task than when it was done with a perceptual or central

load memory task. Similar logic is applied when examining the change in

performance produced by an increase in tracking bandwidth.

Combined standardized scores were chosen to test the total resource

competition in dual task conditions. By combining scores in this way, the

measure was purposely made to be insensitive to any differential tradeoff in

resource allocation between tasks in different conditions. The standardized

scores of all the conditions are listed in the second column of Table.5.

Insert Table 5 about here

Figure 5 represents the interaction between the tracking difficulty and

the Sternberg manipulation. The abscissa shows the effect of increasing

memory search load by imposing demands upon central load (left panel) or

upon response load (right panel). The solid line indicates the effect of

these manipulations on single task Sternberg performance. The dashed line

represents the dual task performance when the Sternberg manipulations were

combined with a low-bandwidth tracking task. The dot-dashed line shows the

dual task performance when these manipulations were combined with a highbandwidth tracking task. Data presented in the top panels were generated by

the weighting function 0.5 * ZRMS + 0.25 * ( ZRT + ZAC ) for dual task

conditions and by the function 0.50 * ( ZRT + ZAC ) for all the single

memory task conditions. This combination rule was chosen because subjects
Table 5 - Performance and subjective workload of each condition in

comparable units - Experiment 1

Condition

Basic Memory

Perceptual Load

Central Load

Response Load

Low-bandwidth Tr

Low + Basic Mem

Low + Perceptual

Low + Central

Low + Response

High-bandwidth

Narrow window

Wide window

High + Basic Mem

High + Perceptual

High + Central

High + Response (HR) .957

Standardized

P

1) -.847 -1

1) -.665

1) .527

1) -.103

) -1.021

,) -.684

) -.505

:) .188

:) .135

) .258

,) -.151

) -.209

!) -.012

') .228

:) • -.829

!) .957

Score

S

.192

.865

.134

.363

.814

.286-

.064

.465

.225

.208

.580

.446

.190

.427

.904

.799

Effect

P

.88

2.50

1.66

.41

.60

.58

.96

1.87

1.48

1.48

.47

.70

.57

1.20

Size

S

1.47

2.01

1.94

-1-.-6-3-

1.47

1.20

1.19

.79

1.56

1.38

1.43

1.35

1.18

1.31

Decrement

P

.424

2.634

1.603

--- .587-

.799

.947

1.698

3.410'

2.312

2.157

.650

.980

.924 -

2.094

Score

S

.333

1.265

.961

.834

.940

1.303

1.068

.753

1.996

1.544

1.130

1.347

1.735

1.641
39

were instructed to give equal priority to each task and to respond as

accurately and quickly as possible. Data depicted in the bottom panels were

generated by the function ZRMS + ZRT + ZAC for dual task conditions and by

the function ZRT + ZAC for all -the single memory search tasks. The reason

for this new combination format will be discussed latter.

Insert Figure 5 about here

It is noted that a good performance in an easy dual task condition

could have resulted from the combination of a very low standardized RMS

«

score with the memory task performance. Therefore, the large intercept

difference between the dashed line (low-bandwidth tracking with a memory

task) and dot-dashed line (high-bandwidth tracking with a memory task) was

expected because the higher standardized RMS errors in the latter

conditions. The effects of tracking baselines were removed in the numbers

shown next to each point. These numbers equalled the combined increment

from both the control tracking task and the basic memory search task. That

is, difference was first obtained for each standardized dependent measure by

subtracting the single task performance from the dual task performance.

Difference standardized scores from all the dependent measures were then

combined. These scores show the same relative relationships between the

slopes as the relationships shown in the figure.

The resource demand of a low-bandwidth tracking task was tested by

examining the interaction, as represented by the difference in the slope

between the solid and dashed lines. If this tracking task imposes

additional demands on a specific stage, performing it concurrently with a

hard memory task which demands the same resources should elevate the
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,,it r,,;.. _ 40

decrements to a greater extent than when it is performed with a basic memory

search task. That is, the slope of the dashed line should be steeper than

the slope of the solid line and the interaction between the two lines should

be significant. The resource demand of increasing tracking bandwidth was

examined by the same logic via the difference in the slope between the two

types of dashed lines (i.e., the low and high bandwidth dual task

conditions).

As shown in Figure 5a, performance of a central load memory task (CM)

was worse than that of a basic memory search task (BM). However, the slope

of single task conditions (the solid line) was steeper than that of dual

task conditions (the dashed line). Following the additive factor logic, it

appears that the low-bandwidth tracking task did not place extra demands on

central.processes.• -The combined, dual, task performance was actually better

than single task performance in the central load condition and the

interaction between the slopes was significant (F(l,14) = 18.34, p - .001).

Results from dual task studies have shown that a tracking task demands

resources for response selection and coordination (see Wickens, 1981 for a

*

review). However, as shown in Figure 5b, the slopes were parallel between

single memory task (the solid line) and dual task conditions (the dashed

line). In other words, adding the low-bandwidth tracking task placed no

extra demands on response resources.

The results shown i.n the top panels could be a consequence of the

weights assigned to different performance measures. To test this

possibility, equal weights were assigned to the three performance measures

and the results are shown in the bottom panels. The interaction between the

solid line (single tasks) and the dashed line (dual task conditions) in

Figure 5c was not significant. This result, once again, indicated that
41

performing the low-bandwidth tracking task concurrently with the central

load memory task did not enhance the demands for central resources. On the

other hand, the effect of dual task loading was enhanced in the response

load, relative to the central load condition (Figure 5d). The interaction

as represented by the difference in the slope between the solid and dashed

lines in Figure 5d was significant (F(l,14) = 11.23, p^ .005).

Neither the effect of dual task loading in cognitive processes nor the

«

dual task effect in response load was enlarged further by increasing the

tracking bandwidth. That is, increasing bandwidth did not produce any

selective effect on the processing stages. The slopes of the dot-dashed

lines in all the panels of Figure 5 were statistically parallel to those of •'

the dashed lines.

Experiment 2. Raw performance and subjective workload .data from Experiment • '•

2 are shown in Table 6. In general, subjects were faster and more accurate

when the processing code of a memory task was spatial than when it was

verbal. The RMS errors were enlarged in both hard single task and dual task

conditions. Employing a predictor display improved second-order tracking'

performance. But, the command display did not aid the tracking performance

as much as anticipated.

Insert Table 6 about here

To test the prior assumption that resource competition was greater

when a tracking task was performed with a spatial than with a verbal memory

task, the additive factors method was again employed. If this assumption is

right, then the dual task effect of performing a low-bandwidth tracking task

in the spatial code condition should be greater than the effect in the
Table 6 - Raw performance and subjective workload of each condition

Experiment 2

Condition RMS RT AC Workload

Verbal Memory

Spatial Memory

Perceptual Load

Low- bandwidth Tr

Low + verbal mem

Low + spatial mem

High-bandwidth - .

High + verbal mern

High + spatial mem

Second-order Tr

Predictor Display

Command Displ ay

Second-order + PM

Predictive + PM

Command + PM

(BM)

(SM)

(PM)

(L )

(LV)

(LS)

(HI).

(HV)

(HS)

(SE)

(PD)

(CD)

0

.169

.209

.206

... - , -311 .

.312

..318

.398

.252

.387

.442

.330

.449

.623

.557

.576

.665

.633

. -. .

.673

.635

.673

.662

.657

94.

97.

96.

93.

97.

94.

96.

94.

95.

95.

686

270

619

527

179

504

508

707

579

289

1.

0.

0.

1.

2.

1.

, I3.

2.

2.

2.

2.

3.

3.

3.

320

790

948

0

180

632

,755

118

638

480

108

168

423

255

265
42

verbal code condition. The average standardized scores (i.e, 0.5 * (ZRT +

ZAC) for all the single memory task conditions and 0.5 * ZRMS + 0.25 * (ZRT

+ ZAC) for dual task conditions) were analyzed to test the total cost in a

dual task condition, independent of the tradeoff in resource allocation.

The standardized scores of all the conditions are presented in the second

column of Table 7 and graphically depicted in Figure 6.

Insert Table 7 & Figure 6 about here

As shown in Figur e 6a, singl e task performance of a spatial code

condition (SM) was better than that of the verbal code baseline memory

condition (BM: the solid line). The advantage of performing the spatial

memory task also held when it was combined wit h a low-bandwidth tracking ' '"'""

 s

task (the dashed line). However, the superiority in the dual task condition

was relatively smalle r than that in the singl e task condition and the slope

of the dashed lin e is less steep than that of the solid line . Decrements

occurred when the low-bandwidth tracking task was added to the spatial

memory task. On the other hand, performance did not deteriorate by

performing the tracking task concurrently wit h the verbal memory task. As

predicted by the multipl e resources model, the effect of dual task loading

was enlarged in the spatial code condition , relative to the verbal code

condition. The difference in the slope between the solid lin e (singl e

tasks) and the dashed lin e (dua l task conditions) was significan t (F(l,14) =

5.83, p *- .05). Increasing bandwidth did not place additiona l demands on a

specifi c processing code, as indicated by the result that the interaction

between the two types of dashed line s was not significant .
Table 7 - Performance and subjective workload of each condition in

comparable units - Experiment 2

Condition

Verbal Memory

Spatial Memory

Perceptual Load

Low-bandwidth Tr

Low + verbal mem

Low + spatial mem (LS)

High-bandwidth

High + verbal mem (HV)

High + spatial mem (HS)

Second-order Tr

Predictor Display (PD)

Command Display

Second-order + PM

Predictive + PM

Command + PM

Standardized Score

(BM)

(SM)

(PM) '

(L )

(LV)

(LS)

(HI)

(HV)

(HS)

(SE)

(PD)

(CD)

P

.052

-.672

-.474

-1.271

-.218

-.568

-.031

.171

-.050

.688

-.527

.599

.695

.138

.643

S

-.682

-1.442

-1.100

-.829

.239

-.176

-.053

.731

.543

.384

.026

.026

.848

.754

.719

Effect

P

.697

.729

2.755-

.193

.406.

3.403

1.298

3.821

.674

.773

.763

Size

S

2.065

1.996

2.605 •

1.633

2.017

2.575

1.450

1.276

1.686

1.725

1.601

Decrement Score

P

.723

.772

3.053"

.270

.513

4.907

1.775

4.677

1.095

1.335

'1.166

S

.907

.655

.671

1.460

1.215

1.316

.986

1.039

1.520

1.536

1.518
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"igure o. Interaction effects found via the additive factors method

Exneriment 2
43

»

The additive factors method was also applied to test the prior

assumption of the effect of a predictor display. It was presumed that this

display demanded relatively more perceptual resources than did an unaided

display because two distinct objects were processed to determine control

inputs. Data, supporting this assumption, are shown in Figure 6b. The

solid line represents single tracking task performance and the dashed line

shows the performance when the tracking task was added to a perceptual load

memory search task.

The benefit of tracking with a predictor display was pronounced in both

single and dual task conditions, but the advantage was relatively larger in

the former than in the latter condition. The slope of the solid line

(single tasks) was much steeper than the slope of the dashed line (dual task

conditions). That.is, adding a'perceptual load memory task had a small

effect on the unaided tracking task. But, performance dropped to a great

degree when the "predictor display" tracking task was performed concurrently

with the perceptual load memory task. The interaction between the two lines

in Figure 6b was significant (F(l,14) = 61.75, p < .0001).

The perceptual demand of the command display was also tested by the

additive factors method. The solid line in Figure 6c represents single task

performance and the dashed line indicates the effect of adding the

perceptual load memory task to the tracking task. There was no main effect

of the display type and thus no benefit from the command display. In

addition, the two lines in Figure 6c are parallel and there was no •

interaction. That is, the command display imposed no greater demands on

perceptual resources than did the unaugmented display.

In summary, the results of the additive factors analyses from

Experiments 1 and 2 indicate that a tracking task (independent of bandwidth)
44

is more spatial than verbal in its processing code and these demands are

more localized in response processes than in central memory processes. The

locus of resource demand of increasing bandwidth appears to be stage-nonspecific. This evidence confirms the previous tentative finding in Wickens

and Derrick's study (1981).

Experiment 3. The Raft data from Experiment 3 are shown in Table 8.

Tracking performance became worse when a basic dual-axis tracking task (BT -

one cursor controlled by one joystick) was combined with any of the memory

tasks. Performance also decreased when the cursor was controlled by two

joysticks (CT) and dropped further when the display was separated (DT - two

cursors tracked by one stick). Reaction times and errors of a basic memory

task generally increased when a tracking task was added to the memory task,

whereas RT was.-also lengthened from .the basic memory tas'k by.increasing ."•••;•

memory load, employing spatial stimuli, and using a speech response.

Insert Table 8 about here

The standardized performance measures were weighted and combined in the

same format as that used in the second experiment. The standardized scores

of all the conditions are shown in the second column of Table 9. The

additive factors method was then applied to localize the processing demands

of dual-axis tracking tasks in .various display'/control formats. Determining

the locus of resources demanded by these dual-axis tracking tasks was

particularly important because there had been no previous evidence to

support the suppositions that (1) separating an integrated cursor into two

distinct error indicators imposes more demands upon perceptual/cognitive
Table 8 - Raw performance and subjective measure of each condition

Experiment 3

Condition

Basic Memory (BM)

Central Load (CM)

Spatial Memory (SM)

Speech (SP)

Basic Dual-Axes (BT)

Separated display (DT)

Separated control (CT).

BT + basic mem (BB)

BT + central load (BC)

BT + spatial mem (BS)

BT + speech (BP)

DT + basic mem (DB)

DT + central load (DC)

DT + spatial mem (OS)

DT + speech (DP)

CT + basic mem (CB)

CT + central load (CC)

CT + spatial mem (CS)

CT + speech (CP)

RMS

,153

.229

.,181 ;.--.

.171

,186

.191

.176

.263

.275

.280

.270

.186 -• •

.196

.196

.195

RT

.482

.590

.543

1.066

.543

.650

.640

1.098

.570

.661

.654

1.124

• .549

.652

.654

1.125

AC

97.185

93.413

96.526

97.768

• •

96.746

92.289

95.821

96.573

96.045

93.068

94.206

94.823

96.713

91.923

95.845

95.999

Work! i

.891

2.10

1.018

.661

1.000

1.630

. 1.546

1.652

3.237

1.888

1.596

2.380

4.360

2.716

2.239

2.356

4.098

2.730

2.118
45

resources, and (2) separating control increases the demands on responserelated resources.

Insert Table 9 about here

If the assumptions are valid, then (1) the effect of display separation

and the effect of loading central processes of the Sternberg memory search

task should be interactive, and (2) control separation and the basic memory

search task (manual responses were required) should compete for responserelated resources. The relevant data from Experiment 3 are displayed in

Figure 7. The abscissa in panels a, b, and c indicates the effect of

changing a processing characteristic of the basic verbal Sternberg memory

search task (BM) by the.processing code (SM), central load (CM)-, and-•'•'-• •'•• •

response mode (manual to speech - SP) respectively. The solid lines

represent the performance under single memory task conditions. The dashed

lines show the performance when a basic dual-axis tracking task (BT) was

added to each of the memory tasks. The dot-dashed lines indicate the

performance of doing one of the memory tasks concurrently with a tracking

task whose display was separated (DT). The x-dashed lines illustrate the

performance when the memory task was combined with a tracking task in which •

the control was separated (CT).

Insert Figure 7 about here

The dual task effect of performing the integrated dual-axis tracking

task was not enhanced in the spatial code or central load condition, as

compared with the basic verbal memory task. Furthermore, separating display
Table 9 - Performance and subjective measure of each condition in

comparable units - Experiment 3

Condition

Basic Memory

Central Load

Spatial Memory

Speech

Basic Dual-Axes

Separated display

Separated control

BT + basic mem

BT + central load

BT + spatial mem

•BT + speech

DT + basic mem

DT + central load

DT + spatial mem

DT + speech

CT + basic mem

CT + central load

CT + spatial mem

CT + speech

Standardized Score

(BM)

(CM)

(SM)

(SP)

(BT)

(DT)

(CT)

(BB)

(BC)

(BS)

(BP)

(DB)

(DC)

(DS)

(DP)

(CB)

(CC)

(CS)

(CP)

P

-.687

-.060

-.489

.459

-.925

.302

-.476

-.569

-.103

-.262

.053

.235

.586

.560

.930

-.442

-.003

-.210

.266

S

-1.453

.015

-.968

-1.616

-.756

-.118

-.231

-.084

.750

.103

-.130

.404

1.198

.584

.342

.389

1.090

.571 .

.230

Effect

P

1.172

.568

2.924

1.873

.764

.563

.662

.885

.526

.709

.692

.972

.699

.342

.428

.552

.416

Size

S

1.776

.622

-.216

2.205

1.518

2.829

1.897

2.553

2.478

2.147

1.788

1.821

2.027

2.147

1.662

1.739

1.757

Decrement Score

P

1.543

.685

5.641

3.680

1.346

.781

1.169

1.439

.802

1.331

1.490

1.938

1.470

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.763

.952

.758

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— — — —Basic dual-axis + Mem

— - — • —Separated display + Mem

— x—x — x—separated control + Mem

BM SP

Figure 7. Interaction effects found via the additive factors method

Experiment 3
46

or control placed no greater demands on the spatial code or the central

processes than did the basic dual-axis tracking task. In other words, the

four lines are statistically parallel in Figure 7a and in Figure 7b.

When the memory task was responded to with speech, a concurrent benefit

was found in some of the dual task conditions. The slope of the dashed line

(integrated dual-axis tracking with a memory task) in Figure 7c is less

steep than that of the solid line (single memory tasks) and the interaction

between the two lines was significant (F(l,13) = 31.65, p <c .0001). The

dashed line did not interact with the dot-dashed line (separated-display

tracking with a memory task) or with the x-dashed line (separated-control

tracking with a memory task). Thus, it appears that neither the display nor

the control separation demanded a greater load from manual resources than

did the integrated tracking task. ' ' •

Rating reliability

Correlations between the ratings collected in Sessions 3 and 4 were

computed across all the subjects and the conditions for each rating scale.

Table 10 shows these correlations for both the ratings recorded at the end

of each trial and the ratings recorded at the end of a session. Although

both post trial and post session reliabilities were fairly high, the latter

ratings were more reliable than the former. For Experiments 1, 2, and 3

respectively, the reliabilities of post trial ratings were 0.67, 0.69, and

0.70. The reliability of post session ratings in all the three experiments

was 0.83.

Insert Table 10 about here
Table 10 - Rating reliability

Overall workload

Perceptual effort

Mental effort

Response effort

Time pressure

Stress

Performance

Overall effort

Excess capacity

Overall

Experiment I

Post trial, Post session

.72 .79

.67 .75

.73 .83

.69 .85

.53 .85

.66 .87

.78 .76

.57 .84

.68 .-.90 .:••• -..-•

.67 .83

Overall workload

Perceptual/Mental effort

Response effort

Time pressure

Stress

Task complexity

Capacity used

Cooper/Harper

Overall

Experiment 2

Post trial, session

.69 .84

.70 .86

.63 .85

.71 .79

- . .68 .88

.68 .73

.77 .84

.70 .85

.69 .83

Experiment 3

Post trial, session

.67 .88

.72 .90

.77 .83

.68 .79

.52 .79

.70 .71

.80 .90

.78 .82

.70 .83
.47

Predicting global workload from specific rating scales

Table 11 shows the correlations between the overall workload ratings

and ratings on every other scale used in the experiment. As shown in this

table, most ratings were highly correlated with the ratings on the overall

workload scale. A multiple regression analysis was conducted to determine

which variables significantly predict the ratings on the overall workload

scale.

Insert Table 11 about here

The BMDP 9R program was used to execute the multiple regression

analysis. The program was written to "estimate regression equation for

"best" subsets of predictor variables" (BMDP, 1981, p. 264). In practice,

this program selects a subset of predictors that account for most of the

variance of the dependent variable. This program starts with a subset of

any one predictor. For each set, the program then selects the next

predictor that significantly maximizes the adjusted R square. The adjusted

R square indicates the amount of variance of the dependent variable

accounted for by the new subset. The program stops selection when adding a

new predictor to the subset does not significantly improve the adjusted R

square. The program executes this analysis for all possible subsets and

selects the best subset for predicting the dependent variable. In addition,

the program also lists the T-value corresponding to the regression weight of

each predictor in a subset. Therefore, the significance of the contribution

of each predictor can be determined.

The results of the analysis are presented in Table 12. In Experiment

1, the combination of four predictors accounted for most of the variance in
Table 11 - Correlations between overall workload ratings and other ratings

Scale Experiment 1 Scale Experiment 2 Experiment 3

PE

ME

RE

TI

ST

PR

OE

EX

.68

.81

.67

.65

.74

•:: .,18:

.68

-.68

P/ME

RE

TI

ST

CM

' ... CA " .

MCH

.86

.78

.80

.73

.73

' -.5.7 .

.82

.90

.46

.54

. .30

.61

-:.. V..39 ,-•; •

.58

PE - perceptual effort, ME - mental effort, RE - response effort

TI - time pressure , ST - stress level , PR - performance

OE - overall effort , EX - execessive capacity

P/ME - perceptual/mental effort, CM - task complexity

CA - capacity used , MCH - modified Cooper/Harper
48

the overall workload ratings (90%). The other predictors did not

significantly improve^ the adjusted R square. These four predictors were

perceptual effort (PE), mental effort (ME), time-demand (TI), and stress

(ST). Among the four predictors, mental effort had the highest regression

weight and accounted for more variance than the other three predictors. In

Experiment 2, workload was best predicted by perceptual/mental effort

(P/ME), time-demand (TI), capacity used (CA), and the modified Cooper/Harper

scale (MCH). These variables accounted for 95% of the variance in the •

overall workload ratings. Ratings on P/ME had a higher regression weight

than the other three predictors. In Experiment 3, 93% of the variance in

the overall workload ratings was accounted for by the combination of

perceptual/mental effort (P/ME), response effort (RE), stress (ST), and task

complexity (CM). The P/ME scale had the.highest regression weight among the

four predictors. Across the three experiments, perceptual/mental effort

scale was the one that consistently predicted the ratings on the overall

workload scale.

Insert Table 12 about here

The perceived structure of subjective difficulty

Similarity judgment data were analyzed by the SINDSCAL program. The

interpretation of the disclosed dimensions was based upon (1) the

correlations with the unidimensional ratings, and (2) task location on each

of the dimensions.

As shown in Table 13, most unidimensional ratings were correlated with

subjective dimensions 1, 3, and 4 in the data of Experiment 1, whereas
Table 12 - Predicting Overall workload from specific scales

Experiment 1

Adjusted R square = 0.90

WK = .095 + .263 PE + .319 ME + .154 TI + .292 ST

Experiment 2

Adjusted R square = 0.90

WK = =.03 + .549-.P/ME-+. .359-:TI:-.10 CA.:+ .13 MCH •

«

Experiment 3:

Adjusted R square = 0.93

WK = .085 + .979 P/ME - .085 RE -.317 ST + .295 CM
49

response effort ratings were related to all of the four dimensions. Data of

Experiment 2 (Table 14) showed that the two subjective .dimensions were

correlated with the following scales: overall workload, perceptual/mental

effort, response effort, task complexity, and the modified Cooper/Harper

scale. The three subjective dimensions disclosed from the data of

Experiment 3 (Table 15) were correlated with overall workload, task

complexity, capacity used, and the modified Cooper/Harper scale. Overall

workload ratings were related to most of the subjective dimensions across

different task configurations.

Insert Tables 13, 14, & 15 about here

Replicating Yeh and .Wickens1 result (1984), a dimension of processing

codes was revealed from the SINDSCAL solution of Experiment 1. ^Figure 8

presents two of the subjective dimensions from Experiment 1. As Shown in

this figure, all the single memory tasks demanding verbal codes had positive

weights on Dimension 2. The two single tracking tasks (low-bandwidth

tracking - L, and high-bandwidth tracking - H) had negative weights. Dual

tasks that demanded both verbal and spatial codes were located between the

two clusters.

Insert Figure 8 about here

Two of the subjective dimensions disclosed from the data of Experiment

1 were associated with the demand on perceptual/central resources. As shown

in Figure 8, the first dimension was related to the processing stage. Basic

rr.emory tasks and perceptual load memory tasks had positive weights on this
Table 13 - Correlations of SINOSCAL dimension weights with

unidimensional ratings (Experiment 1}

Unidimensional scale Dimension

123 4

Overall workload -.75 -.43 -.87 .64

Perceptual effort . -.48 -.41 -.89 .59

Mental effort -.80 -.32 -.85 .58

Response effort -.68 -.57 -.87 - .64

Time pressure • • -'-• • ,: --.71 :• ..'r..§3 . "..1-..87 ' -•• .63

Stress -.77 -.43 -.86 .64

Performance .71 .44 .80 -.74

Overall effort -.71 -.52 ...86 .68

Excess capacity .73 .46 .89 -.62

Note. The following critical values can be used to evaluate the

coorelations reported in the table: r(12) = .532 , p 4. ,05;

r(12) = .612, p -c .01
Table 14 - Correlations of SINDSCAL dimension weights with

unidimensional-ratings (Experiment 2)

Unidimensional scale Dimension

1 2

Overall workload . .70 .76

Perceptual/Mental effort .61 .75

Response Effort .72 .79

Time pressure -72 .76

Stress •:••. - •"

 ; - .50 .74

Task complexity .67 .84

Capacity used .57 .74

Cooper/Harper .67 .79

Note. The following critical values can be used to evaluate the

coorelations reported in the table: r(9) = .605 , p < .05;

r(9) = .685, p z .01
Table 15 - Correlations of SINDSCAL dimension weights with

unidimensional ratings (Experiment 3)

Unidimensional scale Dimension

1 2 3

Overall workload .52 .82 -.77

Perceptual/Mental effort .39 .77 -.81

Response Effort .. ••• . .46 .90 -.70

Time pressure .18 .73 -.84

Stress -.00 .21 -.68

Task complexity .51 .79 -.79

Capacity used .55 '.79 -.79

Cooper/Harper -52 .81 -.78

Note. The following critical values can be used to evaluate the

coorelations reported in the table: r(14) = .497 , p<c .05;

r(14) = .623, p^c .01
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;-'. SI^DSCAI. solution rroir. the ;i~.iliritv ratings of task
50

dimension. Central load and.response load memory tasks had negative

weights. >Figtme"9' r-'ep'resents Dimensions 1 and 3 from the SINDSCAL solution

of Experiment 1. Dual tasks that demanded more resources to coordinate (all

except the condition in which a low-bandwidth tracking was combined with a

basic memory task - LB) had negative weights on Dimension 3. The central

load memory task also had negative weights on this dimension. The other

tasks demanding less cognitive resources had positive weights on Dimension

3.

Insert Figure 9 about here

All the three dimensions disclosed from the SINDSCAL solution in the

data of Experiment 3 were related to the demands on perceptual/central

resources. Dimension 1 in Figure 10 was tied to the demands for working

memory. All the single memory tasks, which required that information be

held in working memory, had negative weights. On the other hand, single

tracking tasks which did not demand memory capacity were located on the top

of the dimension. Dual tasks, combining the two types of tasks, were

located between the two extremes. Dimension 2 (Figure 11) also appeared to

be associated with the demands for working memory. All the single tasks had

negative weights whereas dual task conditions which required executive

management had positive weights on Dimension 2. On D-imension 3 (Figure 10),

tasks that demanded more perceptual/central resources had negative weights.

These tasks included central load memory task, tasks in which subjects

perceived two cursors, and dual tasks in which a tracking task competed with

a spatial memory task. The other tasks had positive weights on Dimension 3.
ORIGINAL PAGE IS

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i i i - - i i 3

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7

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PM . 3

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 L

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- . . ; - . ..'-. •

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LC

14

HR

- .2

- .3

- . 4

13

HC

RM

HORIZONTAL aX

VE.^TICSL RXI5.

DIMENSION

DIMENSION.

I I I

Figure 9. SIN'DSCAL solution from the similarity ratines of -task
51

Insert Figures 10 & 11 about here

Dissociation between performance and subjective measures

Comparable performance and subjective ratings of overall workload from

each condition are presented in Tables 5, 7, and 9 for-the three experiments

respectively. The tasks that subjects performed in each experiment are

listed in the first column and the codes in parentheses are the

abbreviations of each condition. In dual task conditions, the first letter

indicates the type of the tracking task (L - low bandwidth, H - high

bandwidth, B - basic dual-axis, D - dual-axis tracking with display

separated, and C - dual-axis tracking with control separated) and the second

letter represents the type of the memory task (B - basic verbal, P -

perceptual load, C - central load, R - response load, S - spatial code, and

P - speech mode). Comparable performance and subjective workload measures,

derived from the three types of analysis techniques for testing

dissociation, are listed to the right. Since both the effect sizes and

decrement scores are computed as the changes from the baseline condition(s),

scores for the baseline conditions (easy tracking and basic memory search

task) are both zero.

Tables 16 - 19 list the results of the ANOVA's testing of each

dissociation predicted by the proposed theory. In these tables, comparisons

relevant to the tested dissociation are organized in the first column by the

experiment in which each comparison was made. Following each comparison,

the F and P values of the interaction effect in the analysis of each of the

three types of measures are listed.
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DP

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OB CS

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5 CT

CB 10BP

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5

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SP

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VERTICAL RXIS :

DIMENSIO N II I

OIMEN S IO N 'I

?

igure 10. SIN'DSCAL soluticr. rrcrr. ~r.e simiiairv racir.qs of task

c* * '" ^" i "*'U J. ti*/ ~ £ xc 2 IT i_m
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 15

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IS BP

.6 - - . 4 ' -.2 • 01 . .2 .4 .6

- .5 - .3 - . 1

- . 1

DT -.2

2

5

CM _ 5

.1 .3 . .5

CT

S

e SP 4BM

SM 7

1

BT

- . 4

- .6' 1

HORIZONTAL RXIS:

VERTICPL RXIS :

DIMENSION I I I

D I MENSI.QN ! I
. /• • ^- • ; 52

The results listed in these tables are also represented graphically in

Figures 12 - 17. The Y-value of the ending point of each vector in these

figures represents the amount of performance decrements produced by a

specific manipulation. In a single memory task condition, this amount was

derived by averaging the normalized latency and accuracy decrement scores.

In a dual task condition, it was combined from three normalized decrement

scores (0.5 * RMS decrements + 0.25 * (RT decrements + AC decrements)).

This combined decrement score estimates the total cost in a dual task

condition, independent of the tradeoff in resource allocation or between

speed and accuracy. The X-value of the same point shows the amount of

subjective workload increased from the control condition(s), as assessed by

the overall workload scale. Thus, the angle of the vector indicates its

relative influence on performance vs. subjective ratings, whereas the length

represents the total "strength" of a manipulation.

Data will not be discussed figure by figure, but rather in the context

of each source of dissociation.

perceptual vs. Response resources. According to the proposed theory,

the vector for a response load manipulation should point relatively more

toward the performance axis than the vector for a perceptual or central load

manipulation. All of the relevant results from the three experiments are

presented in Table 16. • . . . -.

Insert Table 16 about here

Vectors a, b and c in Figure 12 are three conditions relevant to this

dissociation in Experiment 1. It was anticipated that both the perceptual
Table 16 - Results of the interaction effect in ANOVA tests

Contrast: Perceptual vs. Response resources.

Comparison Standardized Score Effect Size Decrement Score

Experiment 1

PM vs. RM

CM vs. RM

F value P <

0.16

4.34 .06

F value P

3.21 .10

0.26

F value P <

5.84 .03

5.64 .04

Experiment 2

SE vs. HI 2.92 .11

Experiment 3

DT vs. CT 22.67 .0004

0.85

0.494

10.83 .006

59.29 .00001

The effect of the number of display elements

SE vs. PD

CD vs. PD

SE vs. CD

22.28

52.53

2.00

.0003

.00001

.20

2.30

16.78

5.33

.20

.0001

.025

65.17

79.85

0.03

.00001

.00001

_
53

load (Vector a) and central load condition (Vector b) would be further from

the performance axis^than the response load condition (Vector c). In the

case of perceptual load, this was true and the dissociation (PM vs. RM in

Table 16) was significant in the analysis of decrement scores (F(l,13) =

5.84, p< .03). However, Vector b (central load) points to the left and

above Vector c (response load), in contrast to the prediction. Performance

of the central load memory task was much worse than what its subjective

workload would indicate. Statistics of two measures (standardized scores

and decrement scores) supported this reverse dissociation (CM vs. RM in

Table 16).

Insert Figure 12 about here

The processing stage dissociation in the data of Experiment 2 is shown

in Figure 13. The vector for the second-order tracking task (demands stagerelated resources) was predicted to point relatively more toward the

subjective workload axis than the vector for the high-bandwidth tracking

task (demands stage-non-specific resources). However, the difference in

performance between the high-bandwidth tracking task (Vector a) and the

second-order tracking task (Vector b) is greater than the difference between

their subjective workload ratings. That is, performance decrements of the

second-order tracking task were underrated by the subjective workload,

relative to the high-bandwidth tracking task. This dissociation

contradicted the prediction and the earlier finding of Uickens and Yeh' s

study (1983). The dissociation was significant in the analysis of decranent

scores (F(l,14) = 10.83, p -c .006) and close to the significance level by

testing the standardized scores (p •<- .11).
ORIGINAL PAGE IS

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Poor

c

o

e

01

^u

(U•c

OJu

c

t:

01

N

s

o

2

Perceptual load memory task (PM)

Central load memory task (CM)

Response load memory task (RM)

High BW TR without a window (HI)

Low 3W TR + Basic memory task (LB)

Low BW TR + perceptual load (LP)

Low BW TR -f central load (LC)

Low BW TR + response load (LR)

z.asv

Normalized workload increments

Difficult

^igure 12. Performance and subjective workioaa of the conditions in

Experiment 1
54

Insert Figure 13 about here

The dissociation in the data of Experiment 3 was shown when Vectors a

and b in Figure 14 were compared. Vector a (DT) represents the effect of

separating the display and Vector D depicts the effect of separating the

control (CT) in a dual-axis tracking task. It was predicted that Vector b

should point closer to the performance axis than Vector a. However, the

result showed a contrary effect. Performance of the tracking task with

separated control was much better than that of the same task with separated

display. However, this advantage was underestimated by the difference in

subjective workload ratings and the interaction was significant in the

analysis of both standardized scores and decrement scores (DT vs. CT in

Table 16).

Insert Figure 14 about here

In summary, only one paired comparison confirmed the prediction whereas

the others showed a dissociation in contrast to the prediction. However, it

should also be recalled that the additive factor logic failed to indicate

any differential effect of the two tracking manipulations on perceptual and

response load.

The number of display elements. Increasing the number, of display

elements is a manipulation that was predicted to increase the

perceptual/cognitive load. This increase was predicted to be weighted

heavily in the overall measure of subjective workload even if the added
Poor

G

5-1

U

CJ

di

O

«J

' W

O

it-i

ij

0

a

0)

N

m

E

i-i

O

High BVJ TR (KI)

Non-augmented second-order TR (SE)

Predictor display (PD)

Command display (CD)

Low BW TR + verbal memory task (LV)

Low BW TR + spatial memory task(LS)

High BW TR + verbal memory (HV)

High BW TR + spatial memory (HS)

Difficult

Normalized workload increments

rigui'G 13. Performance and subjective workload of the conditions in

Experiment 2
Poor

a: Separated display TR

b: Separated control TR

(DT)

(CT)

c: Basic dual-axis TR + basic memory (BB)

e: Basic dual-axis TR + spatial memory(35)

f: Separated display + basic memory (D3)

h: Separated display + spatial memory (DS)

i: Separated control + basic memory (C3)

k: Separated control + spatial memorv (CS)

c,asy Difficult

Normalized workload increments

?igure 14. Performance and subjective workload of the conditions in

Experiment 3
55

elements actually improved overall task performance. Therefore, the vector

for the unaided second-order tracking task was anticipated to point

relatively more toward the performance axis than the vector for the same

task with a separate predictor. A similar result was also predicted when

the vector for the object-like "command display" was compared with the

vector for the two-element "predictor display".

Vectors for these three second-order tracking tasks are shown in Figure

13 (Vector b for the unaugmented tracking - SE, Vector c for tracking with a

predictor display - PD, and Vector d for the command display - CD). In

supporting the prediction, Vectors b and d (one element) point more toward

the performance axis than Vector c (two elements). It appears that the

predictor display improved tracking performance a great deal but subjective

workload underestimated the amount of the benefit. The .divergence between

Vectors b and c (SE vs. PD in Table 16) was significant in the analysis of

both standardized scores and decranent scores. The divergence between

Vectors d and c (CD vs. PD in Table 16) was also supported by the statistics

of all the three measures.

The command display was designed to integrate the additional control

Information into the cursor itself. It was presumed that this display would

improve the tracking performance without increasing the subjective workload

because there was no increase in the number of display objects. As shown in

Figure 13, there was both a.slight performance improvement and subjective

workload reduction with the latter being greater than the former (compare

vectors b and d). This difference produced a dissociation that was only

significant in the analysis of effect sizes (F(l,14)= 5.33, p^ .05).
56

Single task difficulty vs. dual task competition. It was predicted

that vectors for hard single task conditions would be closer to the

performance axis than vectors for easy dual task conditions. The

experimental comparisons that tested this prediction are listed in Table 17.

Insert Table 17 about here

This dissociation in the data of Experiment 1 is shown graphically in

Figure 12. Vectors a (PM), b (CM), and c (RM) represent the three hard

single memory tasks. Vector d (HI) shows the effect of increasing tracking

bandwidth. Vector g illustrates the result of combining easy dual tasks

(i.e., a low-bandwidth tracking with a basic memory search task - LB). As

predicted, Vector g is further from the performance axis -than any.vector for

the hard single tasks.

The results showed that the subjective workload of an easy dual task

condition was higher than that of a hard single task even if performance was

better in the former than in the latter condition. The statistics shown in

Table 17 backed the findings. In comparing the dual task condition with the

high-bandwidth tracking task (HI vs. LB in Table 17), all the three measures

supported the dissociation between the workload measures. Two of .the

measures (standardized scores and decrement scores) supported the

dissociation when the central load or the response load condition was

compared with the easy dual task condition (CM vs. LB or RM vs. LB). One

measure (standardized scores) supported the dissociation when the perceptual

load condition was compared with the easy dual task condition (PM vs. LB).

The dissociation between the single task difficulty and the number of

concurrent tasks in the data of Experiment 2 is shown in Figure 13. Vectors
Table 17 - Results of the interaction effect in ANOVA tests

Contrast: Single task difficulty vs. Dual task competition

Comparison Standardized Score Effect Size

F value P < F value P <

Experiment 1

PM vs. LB 15.35 .002 1.26

CM vs. LB 54.25 .00001 7.75 .01

RM vs. LB-" 18;40 . -.001. ••-•• • 2:.56 :- .. ;20- '

HI vs. LB 35.85 .00001 16.27 .001

Experiment 2

HI vs. LV 7.15 .02 5.41 .025

HI vs. LS 6.35 .025 4.74 .05

SE vs. LV 10.66 .006 10.52 .005 '

SE vs. LS 7.47 .02 9.62 .005

Experiment 3

OT vs. BB 31.77 .0001 9.19 ,01

CT vs. BB 2.26 .20 6.33 .03

Decrement Score

F value P <

2.62 .15

41.96 .00001

• 23.52!- •-• .'0004 •

197.78 .00001

103.52 .00001

106.32 .00001

59.21 .00001

47.87 .00001

86.26 .00001

5.86 .04
.57

a and b show the effect of increasing bandwidth (HI) and control order (SE)

respectively. Vectors e and f exemplify respectively the result of

performing a low-bandwidth tracking task concurrently with a verbal memory

task (LV) and with a spatial memory task (LS).

In supporting the prediction, Vectors e (LV) and f (LS) are closer to

the subjective workload axis than Vectors a (HI) and b (SE). Thus, given an

equal level of subjective workload, performance of a hard tracking task was

much worse than that of an easy dual task condition. The statistics

presented in Table 17 supported the findings. In comparing an easy dual

task condition (Vector e or f) with a hard single task condition (Vector a

or b), all the three measures supported the dissociation.

In Experiment 3 (Figure 14), the difficulty of a basic dual-axis

tracking task (BT) was manipulated.by separating.the display elements or

control (Vectors a and b respectively) or by adding an easy memory search

task (Vector c - BB). Vectors a (DT) and b (CT) point to the left and above

Vector c (BB), as predicted by the theory. It appears that given an equal

performance level, subjective workload was higher in an easy dual task

condition than in a hard tracking task. In comparing the dual task

condition with the display-separated tracking task (DT vs. BB in Table 17),

all the three measures supported the dissociation. Two of the measures

(effect sizes and decrement scores) supported the dissociation when the

control-separated tracking task was compared with the easy dual task

condition (CT vs. BB in Table 17).

In summary, the dissociation between the single task difficulty and the

number of concurrent tasks was found to be significant in most of the paired

comparisons across the three experiments. The dissociation indicates that

subjective workload of performing an easy dual task combination was much
58

higher than what the performance would indicate, relative to a hard single

task.

Dual task competition for common vs. separate resources. It was

predicted that performance would be adversely related to the amount of

resource: competition whereas subjective workload would be less sensitive to

the difference in the amount of competition. That is, vectors for the high

resource competition conditions should point more to the performance axis

than vectors for the low competition conditions. The results of the ANOVA's

testing of this hypothesis are presented in Table 18 and graphical

presentations of the relevant data are shown in Figures 12 - 16.

.- Insert/Table 18.& Figure -15

 :about here.'••

The dissociations produced by the degree of resource competition in the

data of Experiment 1 are shown in Figures 12 and 15. In Figure 12, Vector h

represents the condition in which a low-bandwidth tracking task was

performed concurrently with a perceptual load memory task (LP). Vector i

depicts the corresponding condition with a central load memory task (LC),

and Vector j with a response load memory task (LR). Vectors 1, m, and n in

Figure 15 represent the three corresponding conditions with a high-bandwidth

tracking task (HP, HC, and'HR). ' ' ' •• •

The tracking task was inferred by the earlier additive factors analysis

to demand response resources. Therefore, it was predicted that the vector

for a dual task loading in the response load condition would point

relatively more toward the performance axis than the vector for the loading

in the perceptual/central condition (j vs. h & j vs. i 1n Figure 12, and n
Table 18 - Results of the interaction effect in ANOVA tests

Contrast: Dual task competition for common vs. separate resource

Comparison Standardized Score Effect Size Decranent Score

F value P< F value P< F value P<

Experiment 1

11.47 .005

16.41 .002

17.54 .001

17.12 .001

LR vs. LP

LR vs. LC

HR vs. HP

HR vs. HC

Experiment 2

LS vs. LV

HS vs. HV

Experiment 3

BS vs. BB

DS vs. DB

CS vs. CB

BS vs. BC

DS vs. DC

CS vs. CC

17.8 .001

2.63 .15

22.07 .0005

1.87 .20

0.50

0.13

2.68 .20

1.33

0.29

12.31 .004

14.82 .002

7.25 .02

1.30

0.35

0.92

0.79

0.03

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ORIGINAL PAGE IS

OF POOR QUALFTY

-iigh 3H TR -f Basic memory task (H3)

-iich 3W TR + perceptual load (K?)

-iigh 3W TR + central load (KC)

-iich 3W TR + response load (KR)

m

X

Difficult

Morr,ali~ec •.•.-orkicad increments

"icure 15. Performance and subjective workload of the conditions in

Exoerime.nt 1 - High bandwidth tracking with a memorv task
59

vs. 1 & n vs. m in Figure 15). As shown in these two figures, the

prediction was confirmed.

The comparison for this dissociation could be made twice, once with the

low-bandwidth tracking task and-once with the high-bandwidth tracking task.

The statistics shown in Table 18 supported the findings. In comparing the

high competition condition with the condition in which the competition was

low in perceptual load (LR vs. LP or HR vs. HP in Table 18), two of the

measures (standardized scores and decrement scores) supported the

dissociation. One measure (decrement scores) supported the dissociation

when the high competition condition was compared with the condition in which

the competition was low in central load (LR vs. LC or HR vs. HC in Table

18).

In Experiment 2, the tracking, task was found to be time-shared'more

efficiently with a verbal memory task than with a spatial memory task, as

predicted by the multiple resources model. Consistent with the proposed

theory, Vectors e and g (a verbal memory task with a low-bandwidth tracking

task and with a high-bandwidth tracking task respectively) in Figure 13 are

closer to the subjective workload axis than Vectors f and h (a spatial

memory task with a low-bandwidth tracking task and with a high-bandwidth

tracking task respectively). Given the same performance level, subjective

workload was higher when a tracking task was performed with a verbal memory

task than with a spatial memory task. This dissociation was significant'-in

the analysis of decrement scores (LS vs. LV and HS vs. HV in Table 18).

The dissociation produced by the difference in the degree of resource

competition in the data of Experiment 3 is graphically shown in Figures.14

and 16. Vectors drawn in various types of dashed lines in Figure 14

represent the comparisons between different dual task conditions whose
60

memory set sizes were the same (tracking task with a basic verbal memory

task vs. with a spatial memory task). Vectors in Figure 16 show the

comparisons between the conditions whose memory set sizes were different

(tracking task with a spatial memory task vs. with a central load memory

task).

In both Figures 14 and 16, vectors drawn in simple dashed lines are the

conditions in which the integrated dual-axis tracking task was performed

concurrently with a memory task (Vector c for basic verbal memory task - BB,

Vector d for central load memory task - BC, and Vector e for spatial memory

task - BS). Vectors in a dot-dashed line (f, g, and h) represent the three

corresponding conditions when the display-separated tracking task was time^-

shared with a memory task. Finally, vectors in an x-dashed line (i, j and

k) depict the-three-'corresponding:conditions when the control-separated^ ' . -.

tracking task was executed concurrently with a memory task.

Insert Figure 16 about here

The results supported the prediction and replicated the results of

'Experiment 2 in which a single axis tracking task was employed. Vectors for

the conditions in which a dual-axis tracking task was combined with a

spatial memory task point relatively more toward the performance axis than

vectors for the conditions in which the task was added to a' basic, verbal

memory task (compare e vs. c, h vs. f, and k vs. i in Figure 14). Vectors

for the spatial conditions also point relatively more toward the performance

axis than vectors for the conditions in which the tracking task was added to

a central load memory task (compare e vs. d, h vs. g, and k vs. j in Figure

16).
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.Normalized workload increments

Difficult

erformance and subjective workload of the conditions in

xperiment 3 - Tracking with a spatial memory task vs.

Tracking with a central load condition
61

It appears that given an equal performance level, subjective workload

was lower when a dual-axis tracking task competed with a spatial memory task

than when the tracking task was performed concurrently with a verbal memory

task.

In Figure 14, the relative relationship between vectors, different in

the amount of resource competition, was found in all three paired

comparisons. However, only one comparison (e vs. c or BS vs. BB) showed a

significant dissociation in the analysis of decrement scores (F(l,13) =

12.22, p.o.005). This dissociation was not found when the tracking task

was in a format of separated display or separated control (f vs. h or j vs.

i in Figure 14). All the three relevant paired comparisons in Figure 16

reached the significance level. In comparing the dual task effect in the

spatial code with the effect in the central load condition (BS vs. BC, DS

vs. DC, and CS vs. CC in Table 18), two of the measures (standardized scores

and decrement scores) supported the dissociation.

In brief, the dissociation produced by the difference in the degree of

resource competition between different dual task conditions was found in

many paired comparisons across the three experiments. The results' indicate

that performance was effected more by the manipulation of resource

competition than was subjective workload.

Jhe effect of resource-investment factors. It was predicted that a

9

motivation factor would improve performance at the cost of increasing

subjective workload. This dissociation is shown by examining the three

vectors drawn in solid lines in Figure 17 which represent the overall

effects of the manipulations. In this figure, Vector d represents a highbandwidth tracking task (HI), Vector e depicts the same task with a narrovi
62

window (MA), and Vector f illustrates the tracking task with a wide window

(WI). In the latter jtwo conditions, subjects were motivated to invest more

resources to reduce the presence of an unpleasant tone. In supporting the

theory, Vectors e and f point more to the subjective workload axis than

Vector d in Figure 17.

The results of the A.NOVA's are listed in Table 19. In comparing the

"narrow window" condition with the high-bandwidth tracking task, all the

three measures supported the dissociation. Two of the measures

(standardized scores and decrement scores) supported the dissociation when

the "wide window" condition was compared with the condition without a

window.

Insert Figur e 17 & Table 19 about here

As shown in Figure 17, Vector e (NA) also points relatively more toward

the subjective workload axis than Vector f (WI). The subjective workload of

tracking with a narrow window was higher than with a wide window even though

the difference in performance was small (performance actually degraded

slightly with the narrow window). This divergence was significant in the

analysis of standardized scores (F(l,14) = 3.81, p< .02).

Examining individual subject 1s'data, it was found that the width of the

window had different effects on the performance of two different classes of

»

subjects. For one group of subjects (which wil l be labelled Group 1),

performance was worse in the "narrow window" condition than that in the-

"wide window" condition . For another group of subjects (Group 2), their

performance was better in the "narrow window" condition than in the "wide

window" condition.
ORIGINAL PAGE IS

OF POOR QUALITY

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High 3W TR without a window (HI)

High BW TR with a narrow window(NA)

High BW TR with a wide window (WI)

Narrow window. Group 1

Wide window, Group 1

Narrow window, Group 2

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Difficult

Performance and subjective workload of the high-bandwidth

cracking conditions in Experiment 1
Table 19 - Results of the interaction effect in ANOVA tests

The effect of resource investment

Comparison Standardized Score Effect Size

F value P < F value P <

Experiment 1

HI vs. NA 40.12 .00001 3.98 .10

HI vs. WI 40.31 .00001 0.12

NA vs. WI

Overall 8.81 .02 0.18

Group 1 . ,.:. 1>33 •:,:•>:•— ••->,- • ..-r.0,.39 •• • -.'

Group 2 11.92 .015 0.96

Experiment 2

Decrement Score

F value P<

33.84 .00001

38.54 .00001

2.88 .15

0.05 . -•• .

9.18 .02

SE vs. PD 22.28 .0003 2.30 .20 65.17 .00001
63

Vectors o and p in Figure 17 represent the "narrow window" and the

"wide window" conditions respectively for Group 1 subjects. The two vectors

have the same slope and there was no dissociation between the two workload

measures. For this group of subjects, they both performed more poorly and

felt more loaded in the "narrow window" condition. Vectors q and r

illustrate the "narrow" and "wide" window conditions respectively for the

subjects in Group 2. Vector r points to the left and above Vector q. For

this group of subjects, their performance was better in the "narrow window"

condition, although they also felt more loaded. Two of the measures

(standardized scores and decrement scores) supported this dissociation for

Group 2 subjects (see Table 19). The- nature of this difference in

dissociation will be treated in the discussion section.

The dissociation between the unaided second-order tracking'(SE) and the

tracking task with a predictor display (PD) described previously in the

context of Vectors b and c in Figure 13, may also be related to the effect

of resource investment. When subjects performed a second-order tracking

task with a predictor display, they were motivated to utilize the additional

display information inherent in the predictor. Therefore, performance

improved a great deal as the result of this display augmentation. However,

because of the greater resource investment required to process the second

display element, subjective workload underrated the benefit of the predictor

display.
64

Discussion

The primary objective of the present study was to identify sources of

subjective.ratings of difficulty and to determine how these ratings differ

from performance. The multiple resources model (Wickens, 1984) was used as

the theoretical framework for the investigation. Based upon this model, the

processing characteristics of different tasks in a multidimensional space

may be identified. Performance and subjective workload measures of these

tasks may then be compared to examine how they relate to each other under

various processing demands and why they dissociate.

Based upon the multiple resources model and previous findings (Wickens

& Derrick, 1981) a theory of'dissociation .was proposed (Wickens -&.Yeh,

1983). According to the theory, dissociation occurs because (1) the demands

on perceptual/cognitive resources, (2) the effect of resource-investment

factors, and (3) the degree of resource competition, are all read unequally

by the two types of measures. In the present study, three manipulations

were employed to place demands on perceptual/cognitive resources. Two other

manipulations were also employed, one to effect resource investment and the

other to effect resource competition. Global workload ratings were used to

represent the aggregate demands from all the components and were then

compared with performance to test the hypotheses drawn from- the proposed •

theory.

The effect of these manipulations on the structure of perceived

difficulty was also disclosed by a multidimensional scaling approach. The

result of this analysis was used to provide converging evidence with

previous findings (Derrick & Wickens, 1984; Yeh & Wickens, 1984) which deny
65

that the two measures dissociate simply because subjective workload itself

is unidimensional, as Gopher and Braune (1983) implied. If the processing

characteristics effect the load experienced by the subjects, the demands on

perceptual/cognitive resources should be unveiled as one of the subjective

dimensions. The regression analysis of the prediction of overall workload

from specific rating scales, was used to present another confirmation that

perceptual/mental effort was an important component of subjective workload.

The information about the components of subjective workload provides

guidance to the workload practitioners concerning what rating scales should

be chosen to assess workload.

When subjects perform a task or dual task combination, both performance

and subjective ratings'may be .used to evaluate the Toad-of-the task imposed-,

upon the human information processing system. Performance, measured by

reaction time, accuracy, or tracking error, directly reflects the internal

nature of the processing characteristics. On the other hand, perceived

workload, estimated by some rating scales, represents the introspection of

the load experienced by the processing system. The processing

characteristics are the common ground that underlies both types of measures

imposed by the task. Processing characteristics, agreed to be

multidimensional, can best be identified by a measure which preserves the

vector properties in the multidimensional space. Performance, a -vector

quantity, has been recognized as such a measure (see Wickens, 1984 for a

review). Subjective ratings can also unveil the multidirnensionality via the

multidimensional scaling approach.

Why would performance and subjective workload dissociate if they both

estimate the load of the same processing characteristics? Could it be that
66

the structure of subjective workload really differs from the structure of

processing resources that underlies the performance? According to Gopher

and Braune (1983), subjective workload follows the pattern of the most

restricted undifferentiated poal while performance follows the pattern of a

multiple resources model. Using the multidimensional scaling approach,

Derrick and Wickens (1981, 1984) have demonstrated that the structure of

perceived task difficulty is related to the structure of processing

resources as portrayed by the multiple resources model. The results of the

present study also supported this corresponding relationship. It appears

that subjective workload is indeed multidimensional although its

multidimensional characteristics are not a direct analog of the

multidimensional characteristics of performance.

Given that-the-structure of subjective-workload corresponds to the r .

structure of processing resources, the multiple resources model provides a

powerful theoretical basis for studying the sources that produce

dissociation between the two measures. Use of the multiple resources model

permitted a manipulation of task difficulty by placing different demands and

competition on certain resource dimensions (e.g., input and output modality,

codes of central processing, or processing stage). In the present study,

the demands of a single task and competition between two tasks were

systematically manipulated employing two types of tasks, tracking and a

Sternberg memory search task. These manipulations were carried out via

different means: (1) excessive loads were placed on a set of resources

(e.g., second-order tracking), (2) the demand on a specific processing stage

was varied (perceptual/central and response load memory task), and (3) the

degree of competition for a processing stage or a processing code was

manipulated in dual task conditions.
67

The additive factors method was used to confirm the processing demands

of various tracking tasks. This Information was then used to verify the

degree of resource competition in different dual task conditions. The

results of the additive factors analysis indicated that (1) A low-bandwidth

single axis tracking task was time-shared efficiently with a verbal memory

task, but not with a spatial memory task or a response load memory task.

Thus, the demands of a tracking task were conclusively verified to be both

spatial and response-related. (2) The manipulation of increasing bandwidth

produced an additive effect (i.e., it did not enlarge the effect of adding a

low-bandwidth tracking task to a memory task). This result replicates the

finding of Wickens and Derrick's study (1981) that increasing bandwidth

demands resources that are not specific to any processing stage or

processing code. (3)' The'1 predict or display, demanded more-perceptual

 : : ' •

resources in comparison with an unaugmented display. (4) Separating the

display cursor or the control input in a dual-axis tracking task did not

impose extra loads on any specific processing stage or code.

MDS results

With the processing demands of the single tasks diagnosed, the

processing characteristics or the interference pattern of a dual task

condition may be understood in terms of the demand and competition on each

resource dimension. This information'provided a basis-for examining the

relationship between performance and subjective workload. Prior to the

investigation of the relationship, a measure was needed to represent the

subjective introspection of workload. The subjective measure of overall

workload was found to correlate with three of the four dimensions in the

data of Experiment 1 (Table 13), with the two dimensions revealed from the
68

MDS data of Experiment 2 (Table 14), and with all the three dimensions

disclosed from the data of Experiment 3 (Table 15). Therefore, this scale

was used to indicate the aggregate weighting on the components in the

multidimensional space and to represent the subjective introspection of the

experienced load. Overall workload ratings of various processing

characteristics were then compared with performance to identify the sources

of dissociation.

According to the proposed theory of dissociation (Wickens & Yeh, 1983),

dissociation occurs because certain demands are read differently by the two

measures. Performance reflects every aspect of the processing dynamics

whereas subjective workload is postulated to be sensitive to the amount of

aggregate resource investment and be dominated by the demands for

perceptual/cognitive-resources.-. The salience of the perceptual/cognitive

demand in subjective introspection was partially supported by the structure

of subjective workload revealed from the data. All the three subjective

dimensions disclosed from the MDS data of Experiment 3 were related to the

demands on perceptual/cognitive resources. Two subjective dimensions (the

processing stage and the demands for perceptual/cognitive resources)

revealed from the data of Experiment 1 were also associated with this

processing characteristic. Moreover, the high regression weights of the

perceptual/mental ratings on the prediction of overall workload also suggest

that the demand for perceptual/cognitive resources is a salient component of

overall workload.

Thus, it is clear that the subjective experience of mental workload is

strongly influenced by the perceptual/cognitive demands. The issue of

whether these demands also represent a source of dissociation with

performance wil l be discussed below.
69

Discussion of dissociations

Three types of manipulations were employed in the present study to

place demands on perceptual/cognitive resources. (1) The number of

concurrent tasks: The difficulty of a single task was increased by adding

another easy task. In such a condition, many subsystems in the

multidimensional space are utilized and executive management is necessary to

coordinate the time-sharing between the two tasks. This cost of concurrence

was assumed to impose an extra load on perceptual/cognitive resources. (2)

The number of display elements: This manipulation was imposed by separating

or integrating the additional predictive control information with the cursor

in a second-order tracking task in Experiment 2. The manipulation on the

number of display elements was also employed by separating the display

cursor of a:dual-axis tracking .task in "Experiment 3. -.-When.-the-number -of- •

elements was greater, the display required more perceptual/cognitive

resources. (3) Perceptual/cognitive difficulty: The difficulty of a basic

Sternberg memory search task was increased in Experiments 1 and 3 by placing

an extra load on the perceptual or central stage of processing in order to

elevate the demands on these resources.

Some dissociation effects between performance and subjective workload

were found as the consequence of the different demands on

perceptual/cognitive resources imposed by different manipulations.

(1) Single task difficulty vs. dual task competition. According to the

proposed theory, both manipulations will elevate subjective workload and

damage performance. However, performing two concurrent tasks will place an

additional load on perceptual/cognitive resources because the executive

management is necessary to process and coordinate two tasks. Hence,
70

subjective workload will overestimate the performance decrements in easy

dual task conditionst relative to difficult single task conditions.

Replicating previous findings (Derrick & Wickens, 1984; Wickens & Yeh,

1983), a strong dissociation was found. As summarized in Table 17, this

dissociation was shown in various comparisons across different task

configurations and across tnt three types of methods from which comparable

measures were derived. Performance decrements were lower when doing a hard

single task than when doing an easy dual task combination, but subjective

workload was higher in the latter condition.

(2) The number of display elements. According to the theory,

increasing the number of display elements will impose more demands on

perceptual/cognitive resources and hence elevate the subjective workload.

The effect of the predictor -display was one-example of this dissociation.

When a second-order tracking task was performed with a predictor, two

elements (cursor and predictor) were processed and coordinated. Therefore,

demands on perceptual/cognitive resources were higher and subjective

workload underrated the benefit of the predictor on the tracking

performance.

The weak dissociation produced by the command display was also in

accord with the effect of the number of display elements. Performance

improved slightly, but not significantly with the command display which did

not include a separate display element. However, the control information

was provided by the integrated object-like symbol which reduced the

requirement of generating lead. Thus, the demands for perceptual/cognitive

resources decreased and subjective workload was lower in the command display

condition than what its performance would indicate.
71

One dissociation was found to be contrary to the prediction. In

Experiment 3, two display elements were processed in the display-separated

dual-axis tracking task. On the other hand, only an integrated cursor was

processed in the control-separated tracking condition. The results from

Experiment 3 indicated that subjective workload was similar between these

two conditions even though performance was worse in the display-separated

condition. This contrary dissociation may result from the lack of

specificity of resource demands of the display-separated condition. The

results of the additive factors analysis showed that unlike the predictor

display, the separate tracking cursors placed no more demands on

perceptual/cognitive resources than did an integrated display. Therefore,

whether the number of display elements will produce a dissociation may

depend upon .whether, .the manipul ation imposes_.extra Demands,.on -, \ ..-•.•- •. ;.. .

perceptual/cognitive resources. Further research is needed to

systematically investigate the effect of integral/separable perceptual

features on the relationship between performance and subjective workload

measures.

(3) Perceptual/cognitive difficulty. Imposing demands on the

perceptual/central resources and on the response-related resources of a task

should equally effect performance, but the former was predicted to affect

subjective workload more than the latter manipulation. Unlike the previous

two manipulations, this-one-failed to provide conclusive evidence for the

dissociation.

The only evidence for this dissociation was observed when the

perceptual load condition was compared with the response load memory task in

Experiment 1. A reverse dissociation was shown when the central load

condition and response load memory task were compared (i.e., subjective
72

workload underrated the performance decrements in the central load memory search

task). This reverse effect may result from the high cognitive load in the

central load memory task in which three sets of alpha-numeric strings were

held in working memory. Under -a high cognitive load condition (Eggemeier

et. al., 1982), subjective workload is a less sensitive measure than

performance.

In Experiment 2, subjects underestimated the performance decrements of

the second-order tracking task, in comparison with the high-bandwidth

tracking task condition. This dissociation is in opposition to the previous

finding. In interpreting the previous result, Wickens and Yen (1983)

suggested that subjective workload was higher for the second-order tracking

task than what its performance would indicate because this task required

subjects to.predict the .acceleration and-hence required more • '

perceptual/cognitive resources. The conflicting results between the two

studies may be due to a difference in procedure across the two studies. In

the previous study, the system gain was the same in both the low and high

order conditions, but in the present study the system gain of the secondorder tracking task was much lower than that of the high-bandwidth tracking

condition. The magnitude of control inputs must be very large in order to

compensate for the low gain in the second-order tracking task and hence

require subjects to allocate more response resources to the task. Future

research is necessary to resolve this conflicting evidence.

The MDS data of Experiment 1 showed that the resource dimension of

processing stages was one of the subjective dimensions of task difficulty

(Dimension 1 in Figure 3). This information, combined with the results of

manipulating the demands on stage-related resources, suggests that the

demands on different processing stages may be weighted equally in subjective
73

introspections of workload. This finding contradicts the proposed theory in

which two basic assumptions were made: (1) subjective introspection reflects

information heeded in working memory, and (2) working memory is represented

primarily by the perceptual/central resources. Based upon these two

assumptions, the theory predicted that the demands on response resources

would not be accurately read by subjective ratings.

Results from dual task studies suggest that the validity of the second

assumption remains uncertain. Wickens (1978) reviewed many dual task

studies and pointed out that "short-term memory processes draw upon a

general resource pool that is available to and used by all concurrent tasks,

whatever their modality and processing stage " (p248). Therefore, the

demands on the response resources, in addition to the demands on

perceptual/central resources,- may .be heeded .in worki ng- memory and

 rhencev~

reflected in subjective introspection of mental workload. Dissociation

between the two workload measures may occur only when the aggregate resource

demands' in working memory are different between the two tasks.

In addition to placing demands on perceptual/cognitive resources, two

manipulations were executed to investigate the other two sources of

dissociation proposed by the theory. (1) Resource-investment factors:

Intrinsic task-related features were employed to induce more resource

investment through an imposition of a predictor element.on a second-order

tracking task in Experiment 2 and an imposition of an error window on a

high-bandwidth tracking task in Experiment 1. (2) Dual task competition for

common vs. separate resources: Competition for a processing stage or

processing code was manipulated to produce different degrees of resource

competition in various dual task conditions.
74

Dissociation was found as the result of both manipulations.

(1) Resource-investment factors. Factors which induce more resource

investment, in order to improve performance, were predicted to increase

subjective workload. On the ottier hand, factors that reduce the amount of

invested resources will decrease subjective workload but deteriorating

performance. In the present study, two types of factors were used to induce

greater investment of resources. The first one was the predictor display in

a second-order tracking task. By offering more precise information, this

predictor display would lead the subjects to use that information and hence

invest more resources in the processes. On the other hand, superior

performance would occur as the benefit of the more precise information.

This effect in fact was obtained. The dissociation was confirmed when the

"predictor displ-ay"* cond-ition. was compared with the urtaugmented display.

Motivational variables are another type of factor that may induce

greater investment of resources. Using pay bonus as an incentive, Vidulich

and Wickens (1983) demonstrated that an external motivational factor may

produce this sort of dissociation. Under the bonus condition, subjects'

performance was better than that under the no bonus condition. But,

subjects felt equally or more loaded when doing the task in the bonus

condition. Tulga (1978) showed that a subjective variable could also

produce dissociation. When the load exceeded the information processing

ability, subjects-could not. reach the objective demand. When the

discrepancy between the actual performance and the objective criterion was

large, subjects tended to lower their performance criterion. The change of

performance criterion produced worse performance but lowered the perceived

workload.
75

In the present study, another type of motivational variable was

employed via intrinsic task-related features. A viindov* (independent of the

width of the acceptable error range) in a high-bandwidth tracking task

induced the subjects to invest more resources to reduce the presence and

duration of an unpleasant tone. Greater investment of resources under a

"window" condition resulted in superior performance in comparison with the

"no-window" condition and dissociation between the two workload measures

occurred.

The difference between Group 1 and Group 2 subjects indicated that

there were individual differences in fulfilling the objective demand. In

general, Group 2 subjects performed better than Group 1 subjects in the 2ndorder tracking tasks. Group 2 subjects (better trackers) invested more

resources to match their performance with the objective demand in. the' - * .-'

"narrow" window condition. Their performance was better than the tracking

performance in the "wide" window condition, but they felt more loaded.

Group 1 subjects (worse trackers), in contrast, gave up trying to

achieve the objective demand and lowered the performance criterion in the

"narrow" window condition. Like Group 1 subjects, they felt more loaded in

the "narrow" than in the "wide" window condition, but unlike Group 1, their

performance was worse in the narrow window condition. The association

between the workload measures for this group of subjects was a phenomenon

different from what was observed by Tulga (1978). In Tulga's study,-

performance deteriorated when subjects gave up achieving the objective

demand but the subjective workload decreased.

All the subjects felt more loaded under the narrow window condition

than under the wide window condition for two reasons: (1) The degree of

required precision influences subjective workload (Moray, 1982). In the
76

"narrow" window condition, the unpleasant tone was presented more often and

longer than in the "wide window" condition. Subjects were aware that they

were required to invest more resources to meet the objective demand and

hence they reported a higher workload value. (2) The high frustration level

experienced by failing to meet the performance demand in the narrow window

condition.

The dissociation produced by resource-investment factors is actually

the propelling force behind the use of subjective ratings. Many researchers

argue that performance itself does not always convey valid information of

mental workload (Johannsen et. al., 1979; Sheridan, 1981; Moray, 1982).

Subjects may simply put in more effort to prevent performance decrements

resulting from an increase in task load. Therefore, performance may not

show any change with the elevating load. The results of the present study

and the previous ones (Vidulich & Wickens, 1983; Tulga, 1978) suggest that

the dissociation between performance and subjective workload is inevitable

in some conditions. When more resources are induced by an incentive (bonus)

or intrinsic task-related features (predictor and the window), performance

improves at the cost of an increasing subjective workload. When subjects

lose their motivation and reduce the amount of invested resources, their

performance drops but they feel less loaded (Tulga, 1978). When the

required precision level produces a high frustration level, subjects may

feel more loaded even though they lower the performance criterion (Group 1

subjects in the narrow window condition).

(2) Dual task competition for common vs. separate resources. From dual

task studies (see Wickens 1981 for a review), it has been shown that

performance is adversely effected by the amount of resource competition

between tasks. However, independent of the amount of resource competition,
77

the total demand for all resources or the function of the executive

management may be constant. Therefore, subjective workload, being more

sensitive to the aggregate resource investment, was predicted to

underestimate the difference in performance decrements between dual task

conditions with competitive or separate resource demands.

Results of previous studies (Derrick & Mickens, 1984; Wickens & Yeh,

1983) showed that a strong dissociation occurs when there is a salient

difference in the amount of resource competition between two dual task

conditions. When a tracking task was time-shared with an auditory task

(demands are spread well over many resources), performance was better than

when the task competed with itself (maximum competition). However, :

subjective workload was not sensitive to such a difference in resource

competition. . . - - . . . .• : _>-. • -. -. : . • .:-•

In the present study, both a tracking task and a memory search task

were performed in all the dual task conditions. The competition was

restricted to the processing stage (Experiment 1) and the code of central

processing (Experiments 2 and 3). Therefore,.the difference in the amount •

of resource competition was less salient than the manipulations in the

previous studies. Nevertheless, dissociation was still found in many task

comparisons. Independent of the locus of competition (processing stage or

code), performance decrements in the high resource competition conditions,

in comparison with the -low competition conditions, were'greater but •

underrated by subjective workload estimates.

Strength of dissociations

Among the five aspects of dissociations, some are stronger than the

others and are found in many paired comparisons across the three experiments
78

and across the three methods from which comparable units are derived. The

ratio (AD/PD) of the number of significant dissociations actually observed

to the number of potential dissociations (number of comparison * 3 types of

methods), was quite different among the five different dissociation

phenomena. .The strongest dissociation occurs between the single task

difficulty and the dual task combination, whose AD/PD is 26/30. The second

strongest one is the dissociation produced by the resource-investment

factors (AD/PD is 7/9). The dissociation produced by the degree of resource

competition is found at 15/36. In all three cases, it is important to note

that none of the paired comparisons refute the prediction (i.e., showed a

significant dissociation in the opposite direction).

The AD/PD ratio produced by the number of display elements is 6/12.

One comparison (dual-axiiS'-tracking-.with-display separated vs. tracking with

control separated) showed a contrary dissociation which may result from the

finding that separate cursors in this task did not place additional demands

on perceptual/central resources. The resource dimension of processing

stages appears not to be a source of dissociation. Two out of 12 potential

dissociations confirm the prediction but 5 out 12 refute it.

In summary, subjective workload, sensitive to the amount of aggregate

resource investment, dissociated from performance when the investment was

increased to improve performance. Dissociation also occurred when two dual

task conditions had different degrees of resource competition. Furthermore,

the demand on perceptual/cognitive resources was a salient component of the

subjective perception of mental workload. When the demand on these

resources was raised by imposing a time-sharing requirement or by increasing

the number of display elements, a dissociation occurred. When the demands

were imposed on different processing stages, the demands on response
79

resources and on perceptual/central resources were read equally by the

performance and subjective workload.

Subjective workload scales

Due to the multidimensionality of the internal processing, it is

plausible that certain vector properties of subjective ratings are not

preserved by a global scale. In the MDS data of Experiment I, overall

workload ratings correlated with three subjective dimensions whereas

response effort ratings were tied to all the four dimensions. This result

suggests that the response effort ratings had indeed picked up some

information that was not registered in the overall workload ratings.

If the response effort ratings did indeed respond to task

characteristics that were not reflected in the global -workload ratings', -it -•

is possible that the former might also show less dissociation from

performance. To test whether the dissociation would diminish by using the

response effort ratings, several comparisons were retested and the data are'

presented in Table 20. Among these comparisons, the weak dissociation in

Experiment 1 produced by loading different processing stages (perceptual

load vs. response load or vector a vs. c in figure 12) lost its

significance. It seems, not surprisingly that the demands of the response

load memory task were tapped by the response effort ratings better than by

the overall workload ratings. .

In contrast, reflecting the overall workload, the response effort

ratings were not sensitive to the competition between two tasks for response

resources. The dissociation produced by the degree of competition for

stage-related resources (low-bandwidth tracking with a perceptual load

memory task vs. the tracking task with a response load memory task) remained
Table 20 - Results of the interaction effect from ANOVA' s on

response effort ratings .

Dissociation phenomenon: Perceptual features affect subjective workload

more than it influences performance

Comparison Standardized Score Effect Size Decrement Score

F value P < F value P < F value P <

PM vs. RM .61 - 1.07 - 2.13

SE vs. PD 24.28 .0002 1.31 - 54.45 .00001

DT vs. CT . . 17.49. ..002 . 1.-.73 - - •' 46.27 1.00001-.

Dissociation phenomenon: Single task difficulty vs. dual task competition

Comparison Standardized Score Effect Size Decrement Score

F value P < - F value P< F value P<

RM vs. LB 26.26 .0002 0.49 - 13.62 .003

HI vs. LB 37.22 .00001 16.79 .001 190.47 .00001

HI vs. LV 2.74 .12 11.44 .001 61.15 .00001

SE vs. LV 5.88 .03 17.52 .001 48.50 .00001

DT vs. BB 18.68 .001 4.38 .05 66.08 .00001
Table 20 (continued) - Results of the interaction effect from ANOVA's on

response effort ratings

Dissociation phenomenon: Dual task competition for common vs. separate

resources

Comparison Standardized Score Effect Size Decrement Score

F value P < F value P< F value P<

LR vs. LP . ., .12.53 ' .005 • 1.18. - ..-;-•. 16.52 -.002

HR vs. HP 16.29 .002 1.03 - 22.99 .0003

HR vs. HC 0.05 - 0.61 - 14.79 .002

Dissociation phenomenon: The effect of a motivational factor

HI vs. MA 27.69 .0001 3.15 .10 25.89 .0005
80

significant. Furthermore, the dissociation produced by the number of

concurrent tasks or by a motivational variable was also significant. Hence,

although response effort ratings do reflect differences in the subjective

workload, these differences are not sufficient to account for the major

sources of performance-subjective workload dissociation.

If it is the case that information on some subjective dimension is not

fully preserved by the global ratings, what scales should be used by

workload practitioners?

The most popular multiscale procedure of measuring subjective workload

is the Subjective Workload Assessment Technique (Reid, et. al., 1981). In

this procedure, rankings are made on three scales: time pressure* stress

level, and effort load. Averaged ranking data are fitted by an additive

combination rule via a conjoint measurement methodology; This rule then '

determines how rankings on the three scales should be combined and the

combined value is used to indicate subjective workload. However, the choice

of these scales is atheoretical and certain problems exist in the procedure

and the logic of this assessment technique (Boyd, 1982).

In theory, resource dimensions could be used to assess subjective

workload since the structure of information processing is the common basis

for both performance and subjective workload. However, the exact structure

of subjective workload depends on the range of processing patterns in the

multidimensional space expanded by the set of tasks employed in a study.

Processing characteristics are more likely to be revealed in the

structure of subjective dimensions when they are very distinguishable among

many tasks in a set. For example, four tasks were employed in Derrick and

Wickens 1 study (1981, 1984): a critical tracking task, a visual search

task, an auditory Sternberg task, and a tone judgment task. In a set of 18
81

tasks (four easy, four hard, and 10 dual task conditions), about half of the

tasks required resources from the visual modality and the other half

demanded auditory resources. Consequently, input modality was one of the

subjective dimensions. The amount of resource competition was also quite

diverse in different dual task conditions. When the tracking task was

paired with itself, the competition was the maximum. When the tracking task

was combined with an auditory Sternberg task or a tone judgment task, the

competition as defined by the multiple resources model was close to zero.

The demands were well spread over input modality (visual vs. auditory),

processing codes (verbal vs. spatial), and response mode (manual vs.

speech). As a result, resource cost and competition was revealed as another

subjective dimension of workload.

In the. context of--other-sets of'tasks (Yeh & Wtckensy -1984)., the. code .

of central processing and the aggregate resource cost (which depended upon

the number of subsystems in the multidimensional space engaged in the

processing, the amount of demands on each subsystem, the locus of demands on

each resource dimension, and the distribution of the competition among the

reservoirs) were found to be the subjective dimensions of task difficulty.

In the present study, the amount of demand for perceptual/cognitive

resources was a major manipulation across different tasks. The outcome, as

expected was that this processing characteristic was disclosed from the MDS

data of Experiments 1 and 3'. Converging evidence from-the previous studies

(Derrick & Wickens, 1984; Yeh & Wickens, 1984) and from the present study

indicates that the demand of a single task and competition between dual

tasks on resource dimensions are the cornerstones of the structure of

subjective workload. Whether a particular resource dimension wil l be

revealed from the data depends upon the processing patterns imposed by the
82

set of tasks employed in a study. Since the exact structure of subjective

workload in a set of tasks can only be disclosed post hoc, how may a

workload practitioner choose the scales a priori? Whic h scales should be

selected to assess the load of -different processing characteristics?

Apparently, a global scale may be used to tap the aggregate demands and

variances of all the components, independent of the structure. In Derrick

and Wickens 1 (1984) study, the perceived effort was correlated wit h all

three disclosed subjective dimensions. In the present study, overall

workload was a scale that correlated wit h most subjective dimensions. The

task complexity scale and the modifie d Cooper/Harper scale also correlated

with all Of the subjective dimensions revealed from the data of Experiments

2 and 3. Ratings on these latter two scales had significan t correlations

with the overall workload ratings -(Table 11). It seems.that any.global

scale may be chosen to tap the aggregate demands. The overall workload or

perceived effort scale is recommended for the hig h face validity.

Should specifi c ratings be collected if the overall workload ratings

encompass the load on all of the components? Since the exact structure of

subjective workload depends on the set of tasks, it is always possible that

specific demands are not full y represented by globa l ratings. The results

from the first two experiments illustrat e this point. The response effort

ratings did not contribute significantl y in accounting for the variance of

the overall workload ratings (Tabl e 12). • However, response effort ratings

correlated wit h the two dimensions disclosed from the data of Experiment 2.

Response effort ratings also correlated wit h all of the four subjective

dimensions from the data of Experiment 1 whereas the overall workload

ratings correlated wit h only three dimensions. When the response effort

ratings were used in the analysis, the weak dissociation produced by
83

comparing the perceptual and response load memory tasks vanished. It seems

that one dimension was better represented by the response effort ratings

than by the overall workload ratings. Therefore, specific ratings may be

useful to tap some demands left out by global ratings.

The choice of specific scales should be based upon the nature of the

important components of subjective workload. The correlations between

subjective dimensions and unidimensional ratings (Tables 13-15) as well as

the results of the regression analysis (Table 12) suggest four scales:

perceptual/mental effort, response effort, stress, and time-pressure. The

first three scales were also found to be salient components of task

difficulty or overall workload in Vidulich and Wickens 1 study (1983). Timepressure ratings were not recommended by Vidulich and Wickens (1983) because

these ratings were.insensitive to the .effect of some .manipulations. •'••';' • • ••'••'

Therefore, perceptual/mental effort, response effort, and stress are

recommended to assess the demands on specific aspects.

Given that the demand for working memory was an important component of

subjective workload, immediate ratings may be subject to some phenomenal

aspects that are task-specific and less stable. Results from a previous

study (Yeh & Wickens, 1984) and from the present study indicated that post

trial ratings were in general less reliable than post session, ratings even'

though both were fairly reliable.

The difference between rating reliabilities may occur because the

effect of specific task features is more likely to diminish within the

context of all of the just-performed tasks. For example, an alpha-numeric

string is particularly easy to remember because it has a special meaning for

a subject. When this subject is asked to rate the load immediately after he

performs the task, his ratings will be very different from the ratings of
84

the same condition with another memory set. On the other hand, ratings

collected within the context of all of the just-performed tasks are more

relative. Subjects have a chance to retrospect the experiences of doing

each task. They may compare the differences in general processing

characteristics and rank order the ratings for each condition. It is not

argued that immediate ratings are hence less valid. It is only suggested

that post session ratings, in the long run, may provide relatively stable

and reliable ratings of the processing characteristics. Immediate ratings

are more vulnerable to specific task features that fluctuate from time to

time. If a workload practitioner is concerned with the relative workload

from operating different systems, ratings within the task context are

recommended.

Conclusion

In conclusion, the results from the present study support the view that

the multiple resources model provides a common theoretical basis for

understanding both the objective and the subjective aspects of mental

workload. Through this theoretical model, processing characteristics can be

diagnosed in terms of the demands on resource dimensions. This knowledge

presents a powerful framework for understanding the relationship between

workload measures and for determining why workload measures dissociate.

The dissociation occurs when demands are read unequally by the two

workload measures. Performance, a vector measure, is influenced by every

aspect of processing. On the other hand, subjective workload is more

sensitive to the amount of aggregate resource investment. Furthermore, the

demand for perceptual/cognitive resources was shown to be a salient
85

component from both the MDS analysis of the hidden structure and the

regression analysis of the underlying components of overall workload.

The executive management engaged in processing and coordinating two

tasks was found to be the most potent factor that drove subjective workload.

A hard single task was favored by subjects even though their performance was

better in a dual task condition. A workload practitioner who relies upon

subjective ratings rather than performance, may be biased to choose a nonoptimal system that requires operators to perform just one task rather than

the system that demands dual task performance.

Factors that induce'different amounts of resource investment were also

a source of a strong dissociation. The effect of these resource-investmentfactors exposes one indispensable dilemma existing in workload measurement.

On the one hand, any factor that discourages resource investment may

decrease subjective workload at the cost of deteriorating system

performance. On the other hand, encouraging better performance may increase

subjective workload. Furthermore, intrinsic task-related features that aid

performance will also elevate subjective workload. A workload practitioner

who relies upon subjective ratings, may be led to abandon predictor displays

that produce better performance than conventional displays.

The amount of resource competition may also produce a dissociation.

Dual task performance is adversely effected by the amount of competition

between two tasks, but subjective workload may not show the difference, or

may indicate that the difference is trivial. A workload practitioner should

be aware of this potential dissociation when a system is selected by

subjective workload. Finally, the resource dimension of processing stages,

in contrast, was found not to be a source of dissociation. This resource

dimension was disclosed as one of the subjective dimensions. Moreover, the
86

weak and reverse dissociations suggest that demands for response-related

resources and perceptual/central demands may be equally weighted in

subjective introspection of workload. It is suggested that the demands on

response resources may also be -heeded in working memory and hence be

reflected in subjective workload.

The relationship between workload measures has been a puzzle in

workload assessment. Given the same manipulations, workload measures may

correlate in some conditions and dissociate in other conditions. What is

needed in this field is a theoretical model to understand the perplexing

relationships (Moray, 1982) and how they diversify as the processing

characteristics alter. Derrick's study (Derrick & Wickens, 1981, 1984) is

the first attempt to study the relationship between workload measures from

the framework of the multiple resources model. The studies by Wickens and

Yeh (1983) and Vidulich and Wickens (1983) have confirmed the utility of

such an approach. The present study is a continuing effort to refine our

knowledge of this relationship. Although some aspects (e.g., stress) were

not examined, many aspects of processing characteristics were manipulated to

identify potential sources of dissociation. There is still a need for

research to complete our knowledge and to determine the appropriate methods

for collecting subjective ratings. It is believed that the results from the

present study provide a foundation for future research.
87

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