This study applies the Stochastic Optimized Submovement Model, a psychomotor model of movement, to computer pointing devices. Modeling such movement is necessary in order to build better pointing devices and to explain differences in performance between existing pointing devices. Current Fitts' law research has shown that isotonic devices perform better than isometric devices, but cannot explain why. The reason for this is that Fitts' law only considers overall movement time and distance, and does not consider what happens during movement. Psychomotor models of movement on the other hand, predict the manner in which movement occurs and are based on an analysis of the microstructure of movement.

Six subjects performed 1800 trials of a pointing task with each of two devices, a mouse and an isometric joystick. The microstructure of movement of these trials was compared between devices, within devices, and within subjects. The comparison showed that the force sensitive isometric joystick picks up tremor causing random changes in cursor velocity that make the device difficult to use. Tremor was not observed in mouse trials. A frequency-domain analysis showed that the joystick trials had higher-frequency components than those seen in mouse movement. This finding is consistent with the presence of tremor. Filters can be built that exploit this difference and reduce the effect of tremor.

The microstructure level analysis of joystick trials revealed a pattern of differences between subjects with high perfonnance and those with low perfomance. This suggests that some subjects can overcome tremor with practice. The analysis was also used to test the applicability of the Stochastic Optimized Sub movement model to the two devices. While the model held for the mouse, it could not be extended to the joystick because the tremor obscured the underlying movement.

The study demonstrates the importance of conducting a microstructure level analysis in addition to the standard Fitts' law analysis techniques that are currently used to study pointing devices.