Perceived Time as a Measure of Mental Workload: Effects of Time Constraints and Task Success

The mental workload imposed by systems is important to their operation and usability. Consequently, researchers and practitioners need reliable, valid, and easy-to-administer methods for measuring mental workload. The ratio of perceived time to clock time appears to be such a method, yet mental workload has multiple dimensions of which the perceived time ratio has mainly been linked to the task-related dimension. In this study, we investigate how the perceived time ratio is affected by time constraints, which make time an explicit concern in the execution of tasks, and task success, which is a performance-related rather than task-related dimension of mental workload. We find a higher perceived time ratio for timed than untimed tasks. According to subjective workload ratings and pupil-diameter measurements the timed tasks impose higher mental workload. This finding contradicts the prospective paradigm, which asserts that perceived time decreases with increasing mental workload. We also find a higher perceived time ratio for solved than unsolved tasks, while subjective workload ratings indicate lower mental workload for the solved tasks. This finding shows that the relationship between the perceived time ratio and mental workload is reversed for task success compared to time constraints. We discuss implications for the use of perceived time as a measure of mental workload.