What does Fitts's law predict?

Fitts's law predicts the time to move a pointer to a target as a function of how far away the target is and how large it is. Targets that are closer and larger are faster to acquire; the relationship is logarithmic, captured by an index of difficulty that summarizes the task.

Why do screen corners make good targets?

Because the cursor stops at screen edges, corners and edges act as if they were infinitely large in one or two directions, making them effectively very easy to hit regardless of approach speed. Designers exploit this by placing important controls at corners and along edges.

Pointing and Target Acquisition

Pointing is the act of moving a cursor or finger to select an on-screen target, and target acquisition is the most studied elementary task in interaction, governed quantitatively by Fitts's law.

Definition

Target acquisition is the task of moving an input cursor to a target and selecting it; Fitts's law models the time this takes as a function of the distance to the target and its size, providing a standard basis for designing and comparing pointing techniques and devices.

Scope

This topic covers the design and evaluation of pointing: how Fitts's law predicts movement time from target distance and size, how throughput is computed and used to compare devices, and techniques that improve pointing such as enlarging effective target size or aiding cursor control. It does not cover touch and gesture interaction as a broader modality, treated separately, nor the general predictive modeling framework, introduced under cognitive models of interaction.

Core questions

How does Fitts's law predict pointing time from distance and target size?
How is throughput used to compare pointing devices fairly?
What techniques make small or distant targets easier to acquire?
How is pointing performance measured experimentally?

Key concepts

Fitts's law
index of difficulty
throughput
movement time
effective target width
pointing device
target acquisition
speed-accuracy trade-off

Key theories

Fitts's law: Movement time to acquire a target rises with the logarithm of the ratio of distance to target width, expressed through an index of difficulty; this robust relationship underlies the design and evaluation of nearly all pointing tasks.
Throughput as a device measure: Throughput, derived from the index of difficulty divided by movement time, summarizes a pointing device or technique's efficiency in a way that is comparable across studies and largely independent of the specific targets used.
Empirical device comparison: Classic experiments comparing the mouse with other selection methods established the mouse's effectiveness and the value of controlled pointing studies, a tradition that continues with new devices.

Clinical relevance

Understanding pointing performance lets designers size and place buttons, links, and controls so they are quick and reliable to hit, and lets them evaluate new pointing devices objectively; these insights apply from desktop interfaces to touchscreens and accessibility aids.

History

Fitts established his law of aimed movement in 1954. In the 1970s and 1980s, HCI researchers including Card and English applied controlled pointing studies to compare input devices, contributing to the mouse's adoption. MacKenzie's 1992 work adapted Fitts's law into a standard tool for HCI, including the throughput measure now common in pointing research.

Key figures

Paul M. Fitts
I. Scott MacKenzie
Stuart K. Card
William K. English

Seminal works

fitts1954
mackenzie1992
card1978

Frequently asked questions

What does Fitts's law predict?: Fitts's law predicts the time to move a pointer to a target as a function of how far away the target is and how large it is. Targets that are closer and larger are faster to acquire; the relationship is logarithmic, captured by an index of difficulty that summarizes the task.
Why do screen corners make good targets?: Because the cursor stops at screen edges, corners and edges act as if they were infinitely large in one or two directions, making them effectively very easy to hit regardless of approach speed. Designers exploit this by placing important controls at corners and along edges.