Evaluation of Fitts’ Law Index of Difficulty Formulation for Screen Size Variations

Authors: Hidehiko Okada, Takayuki Akiba

Abstract:

It is well-known as Fitts’ law that the time for a user to point a target on a GUI screen can be modeled as a linear function of “index of difficulty (ID).” In this paper, the authors investigate whether the traditional ID formulation is appropriate independently of device screen sizes. Result of our experiment reveals that the ID formulation may not consistently capture actual difficulty: users’ pointing performances are not consistent among pointing target variations of which index of difficulty are consistent. The term A/W may not be appropriate because the term causes the observed inconsistency. Based on this finding, the authors then evaluate the applicability of possible models other than Fitts’ one. Multiple regression models are found to be able to appropriately represent the effects of target design variations. The authors next make an attempt to improve the definition of ID in Fitts’ model. Our idea is to raise the size or the distance values depending on the screen size. The modified model is found to fit well to the users’ pointing data, which supports the idea. 

Keywords: Fitts’ law, pointing device, small screen, touch user interface, usability.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1337079

References:

[1] P. M. Fitts, "The information capacity of the human motor system in controlling the amplitude of movement,” Journal of Experimental Psychology, vol.47, vo.6, pp.381-391, 1954.
[2] I. S. MacKenzie, "Fitts’s law as a research and design tool in human-computer interaction,” Human-Computer Interaction, vol.7, pp.91-139, 1992.
[3] I. S. MacKenzie, and W. Buxton, "Extending Fitts’ law to two-dimensional tasks,” Proc. CHI’92, pp.219-226, 1992.
[4] M. Oehl, C. Sutter, and M. Ziefle, "Considerations on efficient touch interfaces - how display size influences the performance in an applied pointing task,” in M. J. Smith, and G. Salvendy (eds.), Human Interface, Part I, HCII 2007, LNCS 4557, pp.136-143, 2007.
[5] R. Fujioka, T. Akiba, and H. Okada, "Evaluation of pointing efficiency on small screen touch user interfaces,” in M. J. Smith, and G. Salvendy (eds.), Human Interface and the Management of Information, Part II, HCII 2009, LNCS 5618, pp.375-384, 2009.
[6] M. McClintock, and D. Hoiem, "Minimal target size in a pen-based system,” Abridged Proc. HCI International’ 93, p.243, 1993.
[7] X. Ren, and S. Moriya, "Selection strategies for small targets and the smallest maximum target size on pen-based systems,” IEICE trans. on information and systems, vol.E81-D, no.8, pp.822-828, 1998.
[8] R. L. Potter, L. J. Weldon, and B. Shneiderman, "Improving the accuracy of touch screens: an experimental evaluation of three strategies,” Proc. CHI’88, pp.27-32, 1988.
[9] A. Sears, "High precision touchscreens: design strategies and comparisons with a mouse,” International Journal of Man-Machine Studies, vol.34, no.4, pp.593-613, 1991.
[10] X. Ren, and S. Mizobuchi, "Investigating the usability of the stylus pen on handheld devices,” Proc. 4th Annual Workshop on HCI Research in MIS, pp.30-34, 2005.
[11] R. Plamondon, and A. M. Alimi, "Speed/accuracy trade-offs in target-directed movements,” Behavioral and Brain Sciences, vol.20, no.2, pp.279-349, 1997.
[12] ISO 9241, "Ergonomic requirements for office work with visual display terminals (VDTs) - Part 9: requirements for non-keyboard input devices,” 2000.
[13] T. Kvalseth, "An alternative to Fitts’ law,” Bulletin of the Psychonomic Society, vol.5, pp.371-373, 1980.