Perceived Time as a Measure of Mental Workload: Effects of Time Constraints and Task Success
International Journal of Human-Computer Interaction
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. This study investigates how the perceived time ratio is affected by time
... d 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. A higher perceived time ratio is found 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. A higher perceived time ratio was also found for solved than unsolved tasks, whereas 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. Implications for the use of perceived time as a measure of mental workload are discussed. ABOUT THE AUTHORS Morten Hertzum is associate professor in computer science at Roskilde University. His research interests are in usability evaluation, assessments of how information systems affect human activities and organizational settings, and pilot implementation as a means of learning in information systems development. Currently, much of his empirical work concerns healthcare technologies. Kristin Due Holmegaard is a Ph.D. student in computer science at Roskilde University. Her research interest is methods for the evaluation of usability and mental workload and how these methods interact with contextual variables such as task complexity and time constraints.