Fitt's Law (Pyschomotor Behaviour)

DECEMBER 26, 2004

2.4 Efficiency and Fitts’ Law

“The efficiency of locating and operating screen objects near each other - in this case the navigation menu and scrollbar - as observed by researchers at the NCI and theorized by Nielsen (1999), can be explained by Fitts’ Law, a robust model of human psychomotor behavior developed by psychologist Paul M. Fitts. Essentially, movement time is affected by the distance moved and the precision demanded by the size of the target to which one is moving. Fitts’ Law enables the prediction of human movement and human motion based on rapid, aimed movement, such as found in the use of a computer mouse. Fitts (1954) discovered that movement time is a logarithmic function of distance when target size was held constant, and that movement time is also a logarithmic function of target size when distance was held constant. The time to acquire a target is therefore a function of the distance to and size of the target.

Fitts’ law has been frequently applied to computer interface design (Mackenzie 1992). For all intents and purposes, it simply means that the bigger and closer an item is, the easier it is to click. Position on the screen, then, is a key factor in “ease of click”. In general, shorter mouse movements are better according to Fitts’ law. Therefore, locating the main navigation menu next to the scrollbar on the right side of a Web page should indeed reduce the time required to alternate between the two.” [Source:]