Effects of Gain and Index of Difficulty on Mouse Movement Time and Fitts’ Law

The mouse, being the major means of inputting and controlling data on a computer, should beset right for best performance. Mouse gain, defined as the ratio of the movement distance on the computer screen to the movement distance of the mouse, controls the movement time when both speed and accuracy are important. Reported optimum gain values vary widely from about 2 to 15. An experiment having a targeting task was used to test 16 participants using a wide range of gain (1.2, 2.4, 14.5, and 38.9). An optimum gain of 2.4 was found for a display movement amplitude (A) of 100 mm, whereas the optimum gain was 14.5 for amplitudes of 200 and 400 mm. Depending on the gain and the movement amplitude, Fitts' law should be modified to accurately model movement time (MT) in the form of MT = A + b(ID) - c(A*ID). Only at low gains could a critical index of difficulty be determined, that being about 3.5.