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.