The law describes an average trend, not one operation

What it is

Fitts's law describes how average movement time changes with task difficulty across repeated observations of comparable tasks. It is not a stopwatch formula for one person's one click. A single operation varies with starting posture, attention, motor noise, latency, misses, and hesitation.

Why it happens

The model compresses constraints shared across many movements into a trend line: higher motor difficulty usually requires more average time. Each movement also contains noise and strategy differences—one person may move quickly then correct, another may approach cautiously; the same person changes strategy when tired or distracted. Aggregation reveals the stable structure; a single value cannot.

Studying it

Collect enough repetitions per participant and condition, and decide in advance how to handle practice, errors, and extreme values. Examine distributions, confidence intervals, and residuals by participant and condition as well as the overall regression. For design comparisons, paired conditions and mixed-effects models help avoid treating individual differences as layout effects. A few demonstration clicks cannot establish a law.

Where it stops holding

An average trend does not guarantee equal benefit for everyone. Motor impairments, expertise, hardware, and setting can alter parameters or error patterns. Real tasks with heavy tails from network waiting, occlusion, or interruption should not be folded into pure movement time. Capacity estimates should report ranges, not false millisecond precision.

Applying it

• Use the model to compare the relative motor burden of alternatives, not to promise an exact completion time.
• Gather repeated trials on key paths and inspect medians, distributions, and misses by person and condition instead of a single successful demonstration.
• After release, watch percentiles and failure rates; if the mean improves while some people still miss often, revise hit areas or layout.

Related

• Same group: B1.01.1 Pointing time is jointly determined by target distance and width · B1.01.2 Distance and width affect time logarithmically, not linearly · B1.01.3 Width is the effective size along the movement direction · B1.01.5 Enlarging a target and shortening travel have different costs
• Nearby: B1.06 Memory-scanning law · B1.07 Power law of practice
• Search terms: Fitts's law · movement-time distribution · mixed-effects model