The Case against Secondary Task Analyses of Mental Workload.

In a commonly used sense, mental workload refers to the proportion of an individual's total processing capacity taken up by a particular cognitive task or task combination. One approach to the assessment of mental workload is called the secondary task analysis. In this approach, the operator is required to carry out two simultaneous tasks, assigning one (the primary task) a high priority and the other (the secondary task) a lower priority. The primary task's mental workload is defined in terms of the degradation in secondary task performance occurring under dual-relative to single-task conditions. The validity of this approach critically hinges to the validity of the assumptions (a) that human processing capacity is unitary or undifferentiated; (b) that the human information processing system contains no significant task-specific capacities; and (c) that overall capacity remains invariant across changes in processing demand. The theoretical literature pertaining to these assumptions is reviewed. It is found that while many of the theoretical issues surrounding the assumptions remain unresolved, the available data argue strongly against the general advisability of the secondary task approach. The problem is that the workload ordering obtained by this approach for any set of (primary) tasks can be expected to vary with the secondary task used. Consequently, the approach will not yield a general measure of workload demand. (Author)