Subjects manipulated a control stick to move a cursor displayed on an oscilloscope screen through a distance A to a target of width W. Movement times were found to be a linear function of Fitts' index of difficulty, loga(2A/W). The slope of the Fitts' law relationship was considerably steeper when subjects used a velocity control system rather than a position control system. The intercept of the Fitts' law relationship was higher when a position-plussteadiness criterion was used for determining the end of a movement rather than simply a position criterion. An analysis of the trajectories of the cursor revealed the presence of a series of submovements, and their duration, accuracy, and frequency are systematically related to changes observed in the Fitts' law relationship. Conditions under which Fitts' law may not hold are discussed.Fitts' law states that the time to move a stylus from a home position to a stationary target is proportional to an index of difficulty, logs (2.4/HO (Fitts & Peterson, 1964). A is the distance from the home position to the center of the target, and W

Subjects used either a position or a velocity control system to capture stationary and moving targets. Fitts' Index of Difficulty was found to be a good predictor of capture time for the rate system. However, for the position control system, the Fitts measure failed to predict accurately the capture time for moving targets. An alternate Index of Difficulty measure is proposed which explicitly incorporates a velocity factor and which predicts the overall pattern of capture times for both systems with greater accuracy than Fitts' Index of Difficulty.

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