The law assumes rapid aimed movement; slow tracking and dragging are outside its scope

What it is

Fitts's law addresses a user who knows where a target is and makes a relatively rapid discrete aimed movement to acquire it. Slow continuous tracking, sustained dragging, path tracing, and moving targets entail ongoing control and cannot be explained by endpoint distance and width alone.

Why it happens

An aimed movement has rapid approach plus endpoint correction, with target tolerance as its main constraint. Dragging and tracking require maintaining object, direction, or velocity relations over time; errors can occur anywhere and depend on channel width, latency, dynamic feedback, and inertia. The task structure changes.

Studying it

Segment tasks: onset to first acquisition can be measured as pointing, while held movement along a path should be measured as continuous control. Record time, acquisition, crossings, drops, and corrective trajectories separately. Use a matching paradigm rather than fitting total drag time to a pointing regression.

Where it stops holding

The initial grab and final drop of a drag can still contain local aimed movements, so Fitts's law may describe those stages. The mistake is extending that local model to the full action. Slow motion can also be dominated by search or caution rather than endpoint difficulty.

Applying it

• Split drag design into grab, path, and drop, designing hit area, path tolerance, and landing feedback separately.
• For continuous tasks provide snapping, stable feedback, or recoverable boundary crossing instead of only enlarging the endpoint.
• In testing locate whether failure occurs before press, during movement, or at release, then choose a matching model and change.

Related

• Same group: B1.11.2 Coefficients must be calibrated for device, limb, and population · B1.11.3 The law covers movement time, not target search or deciding where to click · 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: B1.01 Fitts's law · B1.05 Steering law
• Search terms: Fitts's law · aimed movement · continuous control