Fitts' law models movement time as a logarithmic relation to target distance and width

What it is

Fitts' law models target-acquisition movement time as a logarithmic relation to target distance and width: farther, narrower targets usually take longer. A common index of difficulty converts the distance-to-width ratio into a task difficulty in bits, then predicts mean movement time with a linear relation.

Why it happens

Long distances need wider transport; narrow targets need finer endpoint correction. Both raise the speed-accuracy constraint. The logarithmic form says difficulty grows by increments as the distance/width ratio multiplies, rather than proportionally. The model abstracts motor action as information transfer so conditions of varying difficulty can be compared systematically.

Studying it

Create repeated selections across distance and width combinations, recording movement time and endpoints per condition to fit time–difficulty relations. Report the difficulty formula, start/end definition, error treatment, and device mapping. Variants or arbitrary error exclusion change fit and meaning.

Where it stops holding

Fitts' law applies most directly to relatively independent rapid target acquisition, not mechanically to text editing, complex drag, menu search, or multi-stage operations. Shape, direction, occlusion, latency, and practice add factors. It describes an aggregate trend, not an exact time for every individual action.

Applying it

• Use distance and effective width to check whether small, distant, frequent targets create needless difficulty.
• Compare task difficulty and measured time across layout alternatives rather than judging only visual compactness.
• Validate predictions with real error and correction; marked deviation suggests nonmotor factors such as occlusion, semantic search, or latency.

Related

• Same group: C1.15.1 Throughput captures a pointing device's speed-accuracy tradeoff, not speed alone · C1.15.3 Effective width is recalculated from endpoint distribution rather than nominal width · C1.15.4 Throughput across tasks and postures cannot be compared directly · C1.15.5 Mean completion time alone hides speed-accuracy differences
• Nearby: B1 Interaction principles and laws · Q1 Research methods and evaluation
• Search terms: Fitts' law · index of difficulty · target acquisition