Fitts' Law

Fitts's law (often cited as Fitts' law) is a predictive model of human movement primarily used in human–computer interaction and ergonomics. This scientific law predicts that the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target. Fitts's law is used to model the act of pointing, either by physically touching an object with a hand or finger, or virtually, by pointing to an object on a computer monitor using a pointing device.

Fitts's law has been shown to apply under a variety of conditions, with many different limbs (hands, feet, the lower lip, head-mounted sights, eye gaze), manipulanda (input devices), physical environments (including underwater), and user populations (young, old, special educational needs, and drugged participants).

The original 1954 paper by Paul Fitts proposed a metric to quantify the difficulty of a target selection task. The metric was based on an information analogy, where the distance to the target (D) is like a signal and the tolerance or width of the target (W) is like noise. The metric is Fitts's index of difficulty (ID, in bits):

Fitts also proposed an index of performance (IP, in bits per second) as a measure of human performance. The metric combines a task's index of difficulty (ID) with the movement time (MT, in seconds) in selecting the target. In Fitts's words, "The average rate of information generated by a series of movements is the average information per movement divided by the time per movement" (1954, p. 390). Thus,

Today, IP is more commonly called throughput (TP). It is also common to include an adjustment for accuracy in the calculation.

Researchers after Fitts began the practice of building linear regression equations and examining the correlation (r) for goodness of fit. The equation expresses the relationship between MT and the D and W task parameters:

where:

Since shorter movement times are desirable for a given task, the value of the b parameter can be used as a metric when comparing computer pointing devices against one another. The first Human-Computer Interface application of Fitts's law was by Card, English, and Burr (1978), who used the index of performance (IP), interpreted as  ¹⁄_b, to compare performance of different input devices, with the mouse coming out on top compared to the joystick or directional movement keys. This early work, according to Stuart Card's biography, "was a major factor leading to the mouse's commercial introduction by Xerox".

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