The three most common variations of Fitts’
index of difficulty are the Fitts formulation, the Welford formulation, and the
Shannon formulation.A recent paper by Hoffmann critiqued the three and concluded that
the Fitts and Welford formulations are valid and that the Shannon formulation is
invalid. In this paper,we challenge Hoffmann’s position regarding the Shannon
formulation.It is argued that
the issue of validity vs. invalidity is ill-conceived, given that Fitts’ law is
a “model by analogy” with no basis in human motor control. The relevant questions
are of utility: Does a model work? How well? Is it useful? Where alternative formulations
exist, they may be critiqued and compared for strengths and weaknesses, but validity
is an irrelevant construct. In a reanalysis of data from Fitts’ law experiments,
models built using the Shannon formulation are (re)affirmed to be as good as, and
generally better than, those built using the Fitts or Welford formulation.
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R. W. Soukoreff and I. S. MacKenzie, “Towards a Standard for Pointing Device Evaluation: Perspectives on 27 Years of Fitts’ Law Research in HCI,” International Journal of Human-Computer Studies, Vol. 61, No. 6, 2004, pp. 751-789.
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R. J. Teather and W. Stuerzlinger, “Pointing at 3D Target Projections with One-Eyed and Stereo Cursors,” Proceedings of the ACM SIGCHI Conference on Human Factors in Computing Systems—CHI 2013, ACM, New York, 2013, pp. 159-168.
X. Bi, Y. Li and S. Zhai, “Ffitts Law: Modeling Finger Touch with Fitts’ Law,” Proceedings of the ACM SIGCHI Conference on Human Factors in Computing Systems— CHI 2013, ACM, New York, 2013, pp. 1363-1372.
I. S. MacKenzie and R. J. Teather, “Fittstilt: The Application of Fitts’ Law to Tilt-Based Interaction,” Proceedings of the 7th Nordic Conference on Human-Computer Interaction—NordiCHI 2012, ACM, New York, 2012, pp. 568-577.