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.