Monday, October 15, 2007

Fitt's Law

In ergonomics, Fitts's law (often cited as Fitts' law) is a model of human movement which predicts the time required to rapidly move to a target area, as a function of the distance to the target and the size of the target.

Shanon formulation for movement along a single dimension:

T = a + b \log_2 \left(\frac{D}{W}+1\right)


  • T is the average time taken to complete the movement. (Traditionally, researchers have used the symbol MT for this, to mean movement time.)
  • a and b are empirical constants, and can be determined by fitting a straight line to measured data.
  • D is the distance from the starting point to the center of the target. (Traditionally, researchers have used the symbol A for this, to mean the amplitude of the movement.)
  • W is the width of the target measured along the axis of motion. W can also be thought of as the allowed error tolerance in the final position, since the final point of the motion must fall within ± W/2 of the target's centre.

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