UC Irvine Libraries

Taking Fitts’s law as a premise—that is, movement time is a linear function of an appropriate index of difficulty—we explore three issues related to the collection and reporting of these data from the perspective of application within human–computer interaction. The central question involved two design choices. Whether results obtained using blocked target conditions are representative of performance in situations in which, as is often the case, target conditions vary from movement to movement and how this difference depends on whether discrete or serial (continuous) movements are studied. Although varied target conditions led to longer movement times, the effect was additive, was surprisingly small, and did not depend on whether the movements were discrete or serial. This suggests that evaluating devices or designs using blocked data may be acceptable. With Zhai (2004) we argue against the practice of reporting throughput as a onedimensional summary for published comparisons of devices or designs. Also questioned is whether analyses using an accuracy-adjusted index of difficulty are appropriate in all design applications.