The effects of probe length on Fitts' law

Experiments were aimed at determining the effect of a human using an extended probe when making movements that required accuracy at the completion of the movement. Ten subjects performed 64 conditions of varying amplitude of movement, final accuracy and probe length. Movement time increased with probe lengths from 100 to 400 mm for all conditions of amplitude and required accuracy. The effect of probe length was included in a mathematical description of the data through a multiplicative term on Fitts' index of difficulty (ID), possibly arising from tremor of the hand/probe system in controlling approach to the target. Probe length had the greatest effect at high ID, where a probe of 400 mm length increased movement time by about 25% over that for a 100 mm probe. The model developed has application for accurate movements with long tools such as screwdrivers or crowbars, head-mounted devices for the disabled, or other tasks where the work interface is at the end of an extended probe.