Fitts's law
Fitts's law (often cited as Fitts' law) is a descriptive 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).
Original model formulation
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):
