ABSTRACT

Linear describing functions are a mathematical approximation to the regularities in a person’s response in a manual control task. There is also inconsistency or variability in the person’s response as well. If a person is asked to perform the same manual control task several times in exactly the same manner, then the measured performance will not be identical across repeated performances. In other words, there is variability in performance. The chapter focuses on variability in continuous tracking tasks and aims to compare the pattern of variability in continuous tracking with other tasks, such as discrete movements and rhythmic tapping. The rate of change of force variability for a given increase in force magnitude is smaller for larger forces than for smaller forces. Weber’s Law, which is most typically associated with perceptual judgments, has been found to describe the variability in a number of movement tasks—closed-loop tracking, very brief aimed movements, and rhythmic tapping.