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...re a and b are constants determined through linear regression. W corresponds to "accuracy" -- the required region where an action terminates. The log term is the index of difficulty (ID) and carries the unit "bits" (because the base is 2). If MT is measured in "seconds", then the unit for a is "seconds" and for b, "seconds/bit". The reciprocal of b is the index of performance (IP) in "bits/second". This is the human rate of information processing for the movement task under investigation. Variations of the law have been proposed by Welford [15], MT = a + b log2(A / W + 0.5), (2) and MacKenzie [10, 11], MT = a + b log2(A / W + 1). (3) These equations differ only in the formulations for ID. On the whole, Equation 3, known as the Shannon formulation, is preferred because it provides a slightly better fit with observations, exactly mimics the information theorem underlying Fitts' law, and always gives a positive rating for the index of task difficulty. The Geometry of Input Tasks The experiments conducted by Fitts tested human performance in making horizontal moves toward a target. As seen in Figure 1, both the amplitude of the move and the width of the terminating region are measured along th...

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... and Elvers (1988). Performance indices ranged from 1.1 to 5.0 bits/s. There is recent evidence that the following formulation is more theoretically sound and yields a better fit with empirical data (=-=MacKenzie, 1989-=-): MT=a+b log2(A/W+ l). (3) In an analysis of data from Fitts’ (1954) experiments, Equation 3 was shown to yield higher correlations than those obtained using the Fitts or Welford formulation. Another...

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... the cursor (X) to the target (D). The formulas are valid in any dimension. 7The influence of distractors will be studied in future work. 8We chose Fitts’ formulation of the ID, rather than Shannon’s =-=[15]-=-, for the sake of convenience (analytical calculations are thus possible). It should be mentioned that this option has no effect on the bottom line of our argument. CHI 2004s׀sPaper 24-29 Aprils׀sVien...

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...T = a + b (ID) (1) The constants a and b have been described as representing efficiency of the pointing device in question (here, the stylus on a PDA), while the index of difficulty, (ID), defined in =-=[8]-=- as log2(A/W + 1), embodies the intuition that targets are harder to hit the farther they are, but easier to hit the larger they are. Thus the lower a target’s index of difficulty, the easier (faster)...

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... formulation (e.g., Boritz et al., 1991; Gillan et al., 1990). It has also been argued that Fitts, in formulating his model, deviated unnecessarily from Shannon's original work in information theory (=-=MacKenzie, 1989-=-; Shannon & Weaver, 1949), and that a more theoretically sound formulation for the index of task difficulty is ID = log (A /W + 1) (7) 2 In terms of MT, the prediction model becomes MT = a + b log ( +...

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...be predicted using Fitts law [9], relating target width W and target distance (or amplitude) A. The literature argues that such hand movement most closely follows the Shannon formulation of Fitts law =-=[31, 33, 32]-=-: MT = a + b log 2 � A W � + 1 (2.1) where a and b are constants determined by linear regression. The logarithmic term is referred to as the index of difficulty (ID). Welford introduces another model ...

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...portional to the selection difficulty (IDS): IDS = log2 ( k DR +1 ) . (3) ID can be interpreted as bits of information to transmit to the system. The “information theory” interpretation of Fitts’ law =-=[8]-=- states that those bits have to be transmitted through a channel —the movement— with bounded throughput (b) thus leading to MT . The same bits have to be transmitted by our rake cursor technique so we...

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... for one dimension, Fitts’ law [8] predicts the movement time MT to acquire a target of size W at distance A. Typically, MT is the dependent variable in Fitts’ law. In MacKenzie’s Shannon formulation =-=[15]-=-, the law is written ⎛ A ⎞ MT = a + b ⋅log 2⎜ + 1⎟ (1) ⎝W ⎠ In Eq. 1, a and b are empirically determined regression coefficients, which vary among users and devices. The log term is known as the index...