The reach of Fitts' theorem into and beyond the real world


Paul Fitts conducted landmark studies involving upper-limb movements to show how the amplitude of the reaching movement and the width of the target influence the time taken to complete the movements (Fitts 1954: J Exp Psychol; Fitts and Peterson 1964: J Exp Psychol). The formula developed in these studies has been shown to account for relationships between the speed of upper-limb movements and its accuracy. However, one notable problem with the proposed formula is the presumed unitary relationship between amplitude and width manipulations. On one hand, Heath et al. (2011: C J Exp Psychol) reported stronger than expected influences on movement time with amplitude compared to width manipulations. On the other hand, a close re-analysis of Fitts and Peterson indicates stronger than expected influences of width compared to amplitude manipulations. In this presentation, three possible explanations of the discordance between actual and expected influences of amplitude vs. width manipulations on movement time will be discussed. The first explanation is associated with the use of actual vs. effective target width. The second explanation concerns the availability of terminal feedback. The third explanation is related to the actual target sizes employed and their relevance in the real world.