A reappraisal of fitts' law.

This paper presents a reappraisal of Fitts' (1954) law and outlines reasoning which predicts that, for tasks of equivalent difficulty as calculated by Fitts' Index of Difficulty, smaller-tolerance tasks will, within limits, take longer to perform. Reanalysis of Fitts' data appears to confirm this view. The notion that the overall performance of the human motor system plus associated feedback mechanisms can be described by any simple formulation assuming constant weighting of factors over a variety of tasks, such as proposed by Fitts' law, seems to be misfounded. Recent research supports the doubts concerning fixed-parameter linear and nonlinear models of the human motor system. Some implications of these ideas are discussed.