Measurement Invariance of the Nasa TLX

Abstract

Mental workload is one of the most important constructs of interests for Human Factors researchers. Adequately assessing the amount of mental workload that people experience while performing tasks under specific conditions is essential for the design of safe and efficient systems. Due to its ease of use, the NASA TLX has become the most widely used method of measuring mental workload. However, its psychometric properties are still questionable. The purpose of this study was to examine the extent of measurement invariance of the TLX and raise awareness in the Human Factors community. Two hundred participants reported the amount of mental workload they typically experience while driving in urban and rural areas and across the country. Results indicated that the TLX lacked scalar invariance, thereby biasing the estimation of mean scores and making the examination of mean differences misleading. These findings suggest that researchers should first examine the extent of measurement invariance of the TLX before they proceed to make inferences about mean differences in the amount of mental workload reported by participants under different conditions.