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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.

has been cited by the following article:

• TITLE: A Note on the Validity of the Shannon Formulation for Fitts’ Index of Difficulty

  AUTHORS: Ian Scott MacKenzie

  KEYWORDS: Fitts’ Law; Index of Difficulty; Shannon Formulation; Effective Target Width; Scientific Method

  JOURNAL NAME: Open Journal of Applied Sciences, Vol.3 No.6, October 16, 2013

  ABSTRACT: 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 [1] 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.