Three measures of operator mental work load were tested with a large, interactive computer system. Two-operator rating of time stress, and the ratio of the time required to time allowed for each subtask-proved to be significant and easy to use.
An overall indicator, or Figure Of Merit (FOM), of the quality of crew/vehicle system performance is needed to establish the effect of workload on efficiency and to identify overload conditions. A normative FOM is proposed in which performance is measured on a representative task and a normative database obtained. FOMs for subsequent executions of the task are then reported in terms of weighted deviations from average task performance. Performance of discrete tasks is measured primarily in terms of subtask time and errors. Discrete task performance is then combined with a measure of continuous vehicle control. In order to test the normative FOM procedure, the technique was applied to an existing set of data from a simulated landing task in which standard communications with ATC was compared with a data link communications system. The results indicated that while mean task performance was not affected, task variability, as measured by the FOM, was significantly higher when data link communications were used. In order to establish the sensitivity of the normative FOM method, further testing of the measure is recommended.
When faced with decreased time to complete a task, operators often hold performance constant. When workload is excessive, this is not possible and operators may speed work and ignore errors or fail to finish the task. In this experiment, we forced a choice among policies by decreasing time allowed for a task until it could not be completed. Operators established a baseline time to perform a standard task, and times were decreased to 80%, 60%, 50%, 40%, and 30% of this baseline. Operators were able to decrease time per subtask by 20%, but no further decreases were observed as time demands increased. Errors remained constant over all conditions. The data support the view that increases in task demand produce increases in resources that are adequate for small demand increases but insufficient for great task demands.Modem automated systems often require that a variety of tasks be assigned to a human operator, since automation of routine tasks itself creates a need for supervisory functions (Moran, 1983). Since each operator controls a large segment of an automated system's operations, the impact of operator performance is greatly increased. The time an operator takes to perform a task, and the possibility of operator errors, can become important, and possibly limiting, factors in computer systems.
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