A Deeper Look at the NASA TLX and Where It Falls Short

Abstract

The NASA task load index (TLX) is the most used tool for measuring mental workload. The tools widespread use is associated with its pedigree, and its simplicity of application and interpretation. However, the NASA TLX has been criticized for not measuring the mental construct it purports to measure. The particular claim is that the NASA TLX does not measure perceived mental workload, instead it measures perceived task difficulty. We explored this claim using a simple spatial memory task. The results were analyzed with polynomial hierarchical regression. The NASA TLX failed the test of construct validity and, the sub-components each had random interpretations depending on the participant. The second point is particularly troubling, as many system designs could be based on random, non-causal relationships. Both these observations raise doubts in toward the use of the NASA TLX as a calibrated instrument for human factors engineering.