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Th e amount of mental work a person can do in a given time is not a simple quantity to specify. If it is assumed that a person can only do one thing at a time, then every factor that increases the time taken to do a unit task will decrease the number of those tasks that can be done in a given time interval, and thus, decrease the performance capacity. Hence, every factor in interface design might aff ect performance capacity.

Focusing on performance time emphasizes performance measures of workload eff ects. Other impor-tant measures of workload are physiological, such as the rate of secretion of stress chemicals, and sub-jective, such as moods and attitudes. Any factor could be considered a “stressor” if it deteriorates the performance levels, stress hormone secretion rates, or subjective feelings. Th e approach in this section is to indicate some key general topics, rather than to attempt a full review. One can obtain reviews on workload topics in the chapters on fatigue and biological rhythms, pilot performance, and controller performance (Chapters 10, 13, and 19).

Th us, the points made here are concerned with the capacities of diff erent mental processes; extrinsic and intrinsic stressors; individual diff erences; and practical implications.

7.3.1.2.1 Capacities of Diff erent Cognitive Resources

Diff erent aspects of cognitive processing have diff erent capacities. For a review on processing limits, see Sage (1981). Th e capacity of diff erent processes may be aff ected diff erently by diff erent factors. Figure 7.31 shows the time-of-day eff ects on performance in four tasks: serial search, verbal reasoning (working memory) speed, immediate retention, and alertness. Th e diff erent performance trends in these tasks suggest that each task uses a diff erent cognitive resource that responds diff erently to this stress. It is diffi cult to make reliable analyses of these diff erences, but some other tasks in which performance may diff er in this way are coding and syllogisms (Folkhard, 1990).

7.3.1.2.2 Extrinsic and Intrinsic Stressors

Extrinsic stressors are the stressors that apply to any person working in a particular environment, irre-spective of the task that they are doing. Time-of-day, as in Figure 7.31, is extrinsic in this sense. Some other extrinsic stressors that can aff ect the performance capacity are noise, temperature, vibration, fumes, fatigue, and organizational culture.

Intrinsic stressors are factors that are local to a particular task. All the HF/E factors that aff ect the performance speed or accuracy come in this category. Th e eff ect of task diffi culty interacts with motiva-tion. Easy tasks may be done better with high motivation, whereas diffi cult tasks are done better at lower levels of motivation. Th is can be explained by assuming that stressors aff ect a person’s “arousal” level, and that there is an inverted-U relation between arousal level and performance (see Figure 7.32).

Measures of stress hormones and workforce attitudes show that several factors with respect to the pacing of work and the amount of control over their work that a person feels he or she has, can be stres-sors (e.g., Johansson, Aronsson, & Lindström, 1978). Such aspects are of more concern in repetitive manufacturing jobs than in work, such as fl ying or air-traffi c control.

7.3.1.2.3 Individual Diff erences

Individual diff erences aff ect a person’s capacity to carry out a task, and the person’s willingness to do it.

Aspects of individual diff erences fall into at least fi ve groups.

1. Personality. Many personality dimensions, such as extroversion/introversion, sensitivity to stimuli, need for achievement or fear of success, and preference for facts/ideas or regularity /fl exibility, can aff ect a person’s response to a particular task.

Working memory speed

Immediate retention

Overall mean for day

Subjective

“alertness”

Time of day

08 10 12 14 16 18 20 22

Simple serial search speed

FIGURE 7.31 Cognitive processing capacities change during the day. Th e diff erent patterns of change sug-gest that these capacities have diff erent mechanisms. (Reproduced from Folkhard, S., Circadian performance rhythms. In Broadbent, D.E. et al. (Eds.), Human Factors in Hazardous Situations, Clarendon Press, Oxford, 1990, pp. 543–553.)

2. Interests and values. A person’s interests and values aff ect the response to various factors in the task and the organizational climate, which infl uence the willingness and commitment to do or learn a given task. People diff er in their response to incentives or disincentives, such as money, status, or transfer to a job that does not use their skills.

3. Talent. Diff erent people have diff erent primary senses, diff erent cognitive styles, and diff erent basic performance abilities (e.g., Fleishman, 1975). For example, very few of us have the ability to fl y high-speed aircraft .

4. Experience. Th e rest of us may be able to develop higher levels of performance though practice.

Even the few who can fl y high-speed aircraft have millions spent on their training. Th e eff ects of training on cognitive capacities are discussed more in the section on learning.

5. Nonwork stressors. Th ere may be nonwork stressors on an individual which aff ect the person’s ability to cope with work, such as illness, drugs, or home problems.

7.3.1.2.4 Practical Implications

Th ere are so many factors aff ecting the amount of eff ort any particular individual is able or willing to devote to a particular task at a particular time, such that performance prediction might seem impos-sible. Actually, the practical ways of dealing with this variety are familiar. Th ere are two groups of issues, in HF/E design and performance prediction.

Nearly all HF/E design recommendations are based on measures of performance capacity. Any factor that has a signifi cant eff ect on performance should be improved, as far as it is economically justifi able.

Design recommendations could be made with regard to all the intrinsic and extrinsic factors mentioned earlier, and individual diff erences might be considered in selection.

However, it is easier to predict that a design change will improve performance than to predict the size of the improvement. Numerical performance predictions may be made to assess whether a task can be done in the time available or with the available people, or to identify the limits to speed or accuracy on which design investment should best be concentrated. Obviously, it is not practical to include all the possible eff ective factors when making such predictions. Th ree simplifying factors can reduce the

Basic level

of arousal Introverts

Direction of influence of stressor on arousal level

Task difficulty Incentive Noise Fatigue Extraverts

Arousal level

Task performance

FIGURE 7.32 “Inverted U” relation between internal arousal level and performance (it is not possible to account for the eff ect of all stressors in this way).

problem. One is that, although smaller performance changes may give important clues about how to optimize design, from the point of view of performance prediction, these factors may only be important if they make an order of amplitude diff erence to performance. Unfortunately, our data relevant to this issue are far from complete. Th e second point is that only conservative performance predictions are needed. For these purposes, it may be valid to extrapolate from performance in simple laboratory tasks, in which people with no relevant expertise react to random signals, which is the worst case. To predict minimum levels of performance, it may not be necessary to include the ways in which performance can improve when experienced people carry out tasks in which they know the redundancies, can anticipate, and so forth. Th e third point is that, in practice, many of the techniques for performance prediction that have been devised, have the modest aim of matching expert judgments about human performance in a technique that can be used by someone with less expertise, rather than attempting high levels of accuracy or validity.