accuracy, and duration of hand movements, the duration depends on the logarithm of the ratio of amplitude and accuracy. Data presented by A. T. Welford suggest that the logarithmic transform gives additivity of amplitude and accuracy effects. This paper presents data from a variant of Fitts's dotting task, in which amplitude and duration of movements were controlled and accuracy measured. The results support the original ratio equation. Fitts (1954) was the first to suggest an equation which related the amplitude, accuracy, and duration of hand movements. This equation was determined from empirical studies in which S moved his hand to and fro between two targets. The distance (.4) between, and width (W) of, the targets was varied and the time (MT) taken to make the movements was measured. The following equation fitted the data: MT = a-\ogs(2A/W) + b, in which a and b are constants.