The Problem of Consistency in the Design of Fitt's Law Experiments: Consider either Target Distance and Width or Movement Form and Scale

Overview & Synopsis

An intriguing anomaly of the usual way of designing Fitts' law experiments in experimental psychology and HCI is exposed: experiments are traditionally designed so as to carefully balance two ancillary factors, target distance D and target width W, but not task difficulty, the factor unanimously thought to be crucial. Troubling factor confounds and hence quantitative estimation errors result from this inconsistency. The problem, it is suggested, may be fixed if the equivocalness of the fractional expression D/W that appears on the right-hand side of Fitts' law equations is acknowledged. This two-degree-of-freedom expression can be taken to specify either D and W or the form F and the scale S of the movement task. The paper ends up with practical recommendations for the design of consistent Fitts' law experiments. In most cases eliminating one factor will allow a safer estimation of Fitts' law parameters, while simplifying the experimental work.