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## Encyclopedia > Fitts' law

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# Fitts' law

In ergonomics, Fitts' Law (or Fitts's Law) is a principle of human movement formulated in 1954 which states that:

The time to acquire a target is a function of the distance to and size of the target

This can be more rigorously expressed for movement in a single dimension as:

$T = a + b\ log_2 \left(\frac{2D}{W}\right)$

where:

T is the time taken to complete the movement
a and b are empirical constants
D is the distance from the starting point to the center of the target
W is the width of the target

Since the advent of graphical user interfaces, this law is widely quoted in relation to a user aiming to position the mouse pointer on the screen. However, despite the obvious intuitive appeal of the law when applied to this situation, it should be remembered that in its strictest form:

• it applies only to movement in a single dimension and not to movement in two dimensions
• it refers to the simple motor response of, say, the human hand, not to the complex acceleration behaviours of a mouse pointer
• it describes untrained movements, not movements that are executed after months or years of practice

If, as generally claimed, the law does hold true for mouse pointers, one of its consequences would be that the edges and corners of the screen are particularly easy for a user to acquire: because the pointer remains at the edge of the screen no matter how far the mouse is moved, objects at these positions can be considered as having infinite area.

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