Principles of Design #39 – Fitts’ Law

Fitts’ Law refers to the relationship between the time needed to move to a target and the target size and distance. The smaller and more distant the target, the longer it takes to move to a resting position over it. The law also states that the faster the movement and the smaller the target, the greater is the error rate due to the trade off between speed and accuracy. Fitts’ Law is important in the design of controls, layouts and any device which functions to facilitate movement towards a target.

The law mainly applies to rapid pointing movements, and not for continuous movements such as writing and drawing. It is used to predict the efficiency movements for assembly work performed under a microscope as well as movements such as those required to engage a foot to a specific car pedal. The law is the work of Paul Fitts and was first published in 1954.

Pointing movements typically involve one large quick movement towards a target (ballistic movement), followed by fine adjustment movements (homing movements) to a resting position over the target. Homing movements are the main cause of errors and also take the majority of the time required to move.

Designers can decrease errors and improve usability by considering the implications of the law and its applicability to their designs. For example, in using computers, pointing to an object is typically constrained vertically and horizontally, which dramatically increases the speed with which a target can be reached (when it’s located at the edge of the screen), and such constraints are commonly used in applications such as scroll bars but less commonly to the edges of screens which act as a useful barrier to cursor movement. When buttons and objects are placed along the edge or in a corner, this can significantly reduce the time required to access them, resulting in fewer errors and faster working.

A well known example of this is the unintended acceleration problems found in Chrysler Jeep Cherokees in the 1990s, which because of a large transmission hump, had pedals located such that there was a bigger than usual distance between the foot and brake pedal making it difficult to reach, along with an unconventional layout which resulted in drivers pressing the accelerator instead of the brake on some occasions.

To summarise, Fitts’ Law is an important consideration whenever designing for systems that involve pointing. Designers should ensure that controls are near or large (or both), particularly when rapid movements may be required and accuracy is important. Where controls are not frequently used or where they can cause problems if activated, then they should be smaller and more distant. In addition, always consider the use of strategies to constrain movements where this can provide improvements to overall performance and a reduction in error rates.

REFERENCES

Principles of Design, Revised and Updated by William Lidwell, Kritina Holden and Jill Butler (2010)