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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.
Jagacinski et al. (Tue,) studied this question.