Updating Fitts' Law to Account for Small Targets

The use of small display devices is becoming more prevalent in our daily lives, such as smart phones, ultramobile PCs, and even head-mounted displays. Usually, the graphical user interface components shown on these displays are small and often difficult to interact with. Although some prior work has shown that Fitts' law does not hold for small target size, a detailed investigation of Fitts' law in different small target size conditions has yet to be done. In this study we conducted two experiments investigating the impact of small target sizes on pointing performance. We identified a critical point on the target size dimension where Fitts' law started to break down. We then proposed a small target size index of difficulty and an empirical Fitts' formula to extend the Fitts' law beyond the critical point to small target size conditions. Our results show that the new model works well as both a descriptive and a predictive model for small target size conditions. Acknowledgments This work is founded by the National High Technology Development 863 Program of China under the grant NO.2009AA01Z310. The first author is a visiting student at the School of Interactive Computing in Georgia Institute of Technology and gratefully acknowledges the support and facilities provided by Blair MacIntyre and the School. We thank Thad E. Starner for constructive comments on the study and manuscript. We also thank Dongyi Chen, Xinyu Li, Shiji XiaHou, Zhiqi Huang, and Haipeng Wang for their help in data collection. Finally, we thank the reviewers for their suggestions, which greatly helped to improve the manuscript.