Fitts' law is a robust model of human psychomotor behavior developed in 1954. The model is based on time and distance. It enables the prediction of human movement and human motion based on rapid, aimed movement, not drawing or writing.
It seems intuitive that movement time would be affected by the distance moved and the precision demanded by the size of the target to which one is moving. Fitts discovered that movement time was a logarithmic function of distance when target size was held constant, and that movement time was also a logarithmic function of target size when distance was held constant. Mathematically, Fitts' law is stated as follows:
MT = a + b log2(2A/W)
where
Fitts' law is an effective method of modeling rapid, aimed movements, where one appendage (like a hand) starts at rest at a specific start position, and moves to rest within a target area. The law can be used to assist in the design of user interfaces. It can also be used to predict the performance of operators using a complex system, assist in allocating tasks to operators, and predict movement times for assembly line work. However, just as there are advantages, there are also disadvantages.
One such disadvantage is that Fitts' law predicts movement in only one dimension. Fitts' original experiments tested human performance in making horizontal moves toward a target. Both the amplitude of the move and the width of the terminating region were measured along the same axis. It follows that the method is inherently one-dimensional. So, when dealing with two-dimensional target acquisition tasks, new interpretations of "target width" must be considered. Another major deficiency is the absence of a consistent technique for dealing with errors. Researchers have developed a method to handle errors, but it has been largely ignored because of its complexity. Still another disadvantage is a lack of consensus in the measures found in across-study comparisons.
Fitts' law is a powerful method of modeling rapid, aimed movements. It cannot be used to model drawing or writing. The law can be used to assist in user interface design, predict the performance of operators using complex systems, and predict movement time for assembly line work. There are some situations, however, in which Fitts' law cannot be easily applied.
One such area is multi-dimensional target acquisition tasks. Fitts' law is one-dimensional, so his interpretation of target width cannot be used. Another problem arises when researchers attempt to use equations predicted by Fitts' law for various input devices. Different index of performance (IP) measures have been found for the same input device. The problem arises as to which value should be used.