of Hypertext and Graphical User Interfaces

3.2.1 Fitts’ Law

in Vision and Reality of Hypertext and Graphical User Interfaces

In the 1950s the experimental psychologist Paul Fitts has discovered a mathematical relation between the time it takes to acquire a target with the hand and the distance and size of the target area.* * The original papers that present Fitts’ Law are The information capacity of the human motor system in controlling amplitude and movement by Paul Fitts in Journal of Experimental Psychology (Volume 47 p. 381-391, 1954) and Information Capacity of Discrete Motor Responses by Paul Fitts and J. Peterson Ibid. (Volume 67 p. 103-112, 1964). The relation can be expressed as

Tpos = c1 + c2 log2( 2d / w )

where c1 and c2 are device dependant constants. d is the distance to the target of width w.

In the words of Stuart Card, Thomas Moran, and Allen Newell [Card et al. 83, p. 53]:

[The] time to move the hand to a target depends only on the relative precision required, that is, the ratio between the target’s distance and its size.

An obvious conclusion to be drawn for interface design is the rule to make distant items large, e.g. icons and buttons. Or, as Bruce Tognazzini says in Tog on Interface, «close targets are faster to acquire than far ones» [Tognazzini 92, p. 206].
Another direction to exploit Fitts’ Law is to measure and compare the values of c1 and c2 for different graphical input devices. This opens the way to a qualitative analysis of mouse, touch screen, graphic tablet, etc.

Due to Ronald Baecker, Fitts’ Law has first been applied in the field of human-computer interaction at Xerox PARC by Stuart Card, Thomas Moran, Bill English and Alan Newell in the 1970s [Baecker et al. 95, p. 470]. Their studies provided qualitatively founded guidance for interface design and laid the foundations for applied cognitive psychology. Card, Moran, and Newell developed a sound scientific framework that was published as The Psychology of Human-Computer Interaction [Card et al. 83].

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