Workload Transition Implications for Individual and Team Performance (1993) / Chapter Skim
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3 Workload Factors
3 Workload Factors
Pages 54-93
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From page 54... ...
imposed task demands if the difficulty, number, rate, or complexity of the demands imposed on an operator are increased, workload is assumed to increase; (2) the level of performance an operator is able to achieve if errors increase or control precision degrades, workload is assumed to increase; (3)
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As task difficulty increases, performance often, but not always, degrades; response times and errors increase for discrete tasks, control variability and error increase for tracking tasks, and fewer tasks are completed within an interval of time. The workload imposed by one task may interfere with the performance of other concurrent activities.
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suggested that the relationship between workload and performance changes as overall task difficulty is increased. For relatively easy tasks, operators can maintain consistent performance by exerting additional effort if task demands are increased.
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Given the limitations of human memory, vision, physical strength, and so forth, some tasks may stretch or even exceed an operator's capacities; other tasks impose so few demands that they may be performed concurrently with other tasks. Thousands of experiments have been conducted in which a variety of task-difficulty manipulations were imposed to examine different aspects of the human information processing system and to define and quantify the limits of human capabilities.
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Thus, although minimizing error is the goal of most control activities, smoothness and stability may be equally important. Similarly, in discrete control tasks like target acquisition, performance strategies can be described by a tradeoff between the speed and accuracy of the movements.
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A similar relationship has been found between the movement difficulty and subjective workload (Hart et al., 1984b; Johnson and Hart, 1987; Mosier and Hart, 1986~. Instructions to maximize either speed or accuracy influence the workload and performance associated with discrete responses, target acquisition, and continuous control.
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. An operator's ability to respond to changing priorities is influenced by the total demands of concurrent tasks, their resource requirements, instructions, feedback, and training (Tsang and Wickens, 19881.
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Subjects also alternated between the control task and discrete tasks that took several seconds to complete. Response times increased significantly over single-task levels, and subjective workload was high, possibly reflecting the added cost of switching back and forth.
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, thus completing twice as many tasks in the same interval of time as low-scoring subjects, who were more reactive and performed task elements sequentially, even though there was sufficient time to alternate between tasks. In operational situations, team members may request assistance from each other as a strategy for coping with excessive task demands.
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In continuous control tasks, such as that performed by the tank driver, the operator's goal is to minimize the time-averaged difference between a target (e.g., a nominal flight path, a driving lane, a point or line on a screen) and the output of a dynamic system.
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As presentation rate was increased from once every 2.4 minutes to once every 0.8 minutes, subjective workload increased significantly. For complex tasks, the pattern of performance decrements that occurs when there is insufficient time to monitor, process, and respond to all task components depends on the strategy an operator adopts.
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found that map complexity had a more powerful effect on the cognitive operations required for navigation than any other variable; each additional landmark added to the map increased response time by 450 milliseconds. Variability of Task Demands People develop expectations about how much effort a specific task should require.
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For example, Thornton (1985) demonstrated that an unexpected peak of workload introduced in a relatively low workload task, elevated subjective workload for the whole segment, particularly if it occurred late in the interval.
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Even though takeoff and landing segments last only a few minutes and cruise segments last considerably longer, pilot workload ratings reflect the difficulty rather than the duration of a segment (Bortolussi et al., 1987; Hart and Hauser, 1987~. Pepitone et al.
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found that overall workload and subjective fatigue were unrelated to the duration of flights performed at night in the Kuiper Airborne Observatory of the National Aeronautics and Space Administration. Task Requirements and Procedures i It should be clear from the preceding discussion that the introduction of automation must be preceded by a careful analysis of task requirements and human capabilities.
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There is no question that crew workload is influenced by the number and complexity of the information sources that must be monitored and the quality of the information they provide. However, the influence of purely perceptual factors on workload has been examined in less depth than have information processing, memory, and response mechanisms.
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Interface design affects both workload and the cognitive strategies adopted (Woods, 1989~. For example, the design of display elements and their organization can force even a highly skilled user from automatic perceptual processes to effortful cognitive processes, or vice versa.
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Information From the Visual Scene In the real world, operators often rely on visual cues in the external scene (i.e., optical flow, structural transformations, terrain features) to esti mate their orientation, speed, altitude, heading, and location.
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Thus, objects may look very different than they do with the naked eye (Foyle et al., 19901; surface texture and shading are limited or inconsistent with the viewer's expectations; shadows, convex, and concave areas may be either bright or dark depending on the polarity selected; and objects may disappear if their temperature is similar to that of their surround. Again, the perceptual demands thus imposed add significantly to operator workload.
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Because the visual and auditory demands imposed on the operators of many complex systems are so high, and because incoming information may
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Instead, the focus has been on the processes required to interpret, transform, and remember information and formulate a response. Information Processing Variables As the cognitive processing required to interpret incoming information and select a response increases from simple rules of thumb to complex algorithms, inference, or deduction, response time generally increases; errors are more frequent; and experienced workload is higher.
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In fact, subjective workload estimates were often dominated by the difficulty of the skill-based component of the task. Since there is less uncertainty associated with skill- and rule-based behaviors, these tasks are often candidates for automation.
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conducted two simulations of advanced Army scout/attack helicopter missions to evaluate alternative methods of entering discrete commands. Although commands took longer to enter using a voice recognition system than with manual switches (and subjective workload was higher)
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Although humans appear to be aware of the total demands imposed on them (created by variations in task frequency, difficulty, and number) , their estimates of workload are relatively insensitive to resource competition.
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Since memory is fallible, particularly when interruptions occur, written procedures and checklists are used in many operational environments to reduce operator workload and decrease the probability of errors. However, since retrieving the appropriate page from a manual may take an unacceptable amount of time in an emergency situation, critical items (e.g., the first few steps in an emergency checklist)
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(1989) found that response time and subjective workload increased when mental rotation was required to bring two items into alignment for comparison.
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Poorly designed controls, high-order system dynamics, inadequate displays, and incompatible controls and displays may make it difficult for an operator to accomplish even relatively easy tasks. Control Design Many real-world tasks require discrete, transient, periodic, or relatively continuous control inputs in response to direct or indirect feedback from the environment.
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While high-gain systems reduce the workload of the initial phase of target acquisitions (the higher the gain, the smaller the control displacement required) , they impose additional workload during the final phase (the higher the gain, the finer the corrections required to remain over the target area)
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Put very simplistically, position, velocity, and acceleration controls require a minimum of one, two, or three control inputs to bring and hold a reticle over a target. Computer Aiding and Automation Computer aiding and automation are introduced to reduce operator workload, increase safety, improve performance, and extend mission capability.
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Because predictor displays perform some integration and projection for operators, thus allowing them to be proactive rather than reactive, they often improve the accuracy and smoothness of control activities and reduce operator workload. Stability augmentation systems reduce the frequency and order of control inputs required of an operator.
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Often, it simply replaces one form of workload (the physical demands of manual control) with another (the perceptual-cognitive demands of monitoring the system)
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Furthermore, incomplete attention to the current situation and tasks is more likely to result in errors, which will impose additional workload to rectify.
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Pp. 397-416 in Proceedings of the 20th Annual Conference on Manual Control.
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Pp. 100-115 in Proceedings of the 19th Annual Conference on Manual Control.
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Pp. 431-454 in Proceedings of the 20th Annual Conference on Manual Control, Volume 1.
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Pp. 127-149 in Proceedings of the 18th Annual Conference on Manual Control.
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Junker 1979 Effects of simulator delays on performance and learning in a roll-axis tracking task. Proceedings of the Fifteenth Annual Conference on Manual Control.
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Hart, and Y.Y. Yeh 1986 Memory and subjective workload assessment.
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M.A., and C.D. Wickens 1984 Subjective workload assessment and voluntary control of effort in a tracking task.
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Santa Monica, California: Hu man Factors Society. Wiener, E.L., and R.E.
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