Abstract

We address two main issues: the distinction between time-constrained and spatially constrained tasks, and the separable A and W effects on movement time (MT) in spatially-constrained tasks. We consider MT and 3-D kinematic data from human adults pointing to targets in human-computer interaction. These are better fit by Welford's (1968) two-part model, than Fitts' (1954; Fitts & Peterson 1964) ID model. We identify theoretical and practical implications