The law covers movement time, not target search or deciding where to click

What it is

Fitts's law models movement time from action onset to acquisition once a person knows the target location and intended action. Finding the target, reading its label, deciding where to act, and waiting for feedback are other stages. Attributing complete task time to distance and width overstates or misidentifies motor-design effects.

Why it happens

Visual search depends on salience, layout, and attention; choice depends on alternatives, semantics, and skill; movement depends on geometry and control error. Real tasks often pass through these stages in sequence, so a slow earlier stage can hide the benefit of a larger target. Stages can overlap too, so one total-time slope cannot replace process decomposition.

Studying it

Define movement onset, such as first fixation on target, pointer motion onset, or touch onset. Use gaze, trajectories, recording, or staged experiments to measure search, comprehension, choice, movement, and feedback separately. Hold other stages stable when manipulating distance and effective width.

Where it stops holding

In highly practised, fixed-location tasks, search and choice can be small and the movement model explains more variance. In unfamiliar architecture or unclear semantics, movement may barely matter. Stage boundaries are analytical tools, not necessarily conscious discrete user experience.

Applying it

• When work is slow, distinguish cannot-find, cannot-understand, hesitation, miss, and waiting before making buttons larger.
• Lower search and choice cost through layout and wording, then optimise known-target acquisition with distance and hit area.
• Annotate time and errors by stage in tests, confirming a size change improves the motor segment it is supposed to affect.

Related

• Same group: B1.11.1 The law assumes rapid aimed movement; slow tracking and dragging are outside its scope · B1.11.2 Coefficients must be calibrated for device, limb, and population · B1.11.4 Treating index of difficulty as a single quality score is misuse · B1.11.5 “Bigger buttons are always better” ignores finite layout area
• Nearby: B1.04 Hick-Hyman law · B1.01 Fitts's law
• Search terms: Fitts's law · movement time · visual search · choice time