Coefficients must be calibrated for device, limb, and population

What it is

The intercept and slope mapping Fitts's-law difficulty to movement time must be calibrated for the conditions in question. Mice, trackpads, pens, and fingers—and people with different motor ability and experience—have different noise, gain, and strategies. Borrowing coefficients from another device or group does not yield a reliable prediction.

Why it happens

Coefficients absorb device resolution and latency, endpoint variability, visual feedback, control–display mapping, and strategy. A fingertip and mouse cursor have different error structures even at the same index of difficulty; novice and experienced users also differ in onset and correction, shifting or steepening the time function.

Studying it

Collect repeated trials at multiple distances and widths on target hardware, gain, and user population; fit parameters using effective width and movement time. State participant characteristics, device, posture, software version, and error handling. For cross-condition work, compare measured throughput or curves rather than transplanting a parameter.

Where it stops holding

Calibration does not require a personal formula for every user; it prevents unjustified precision. Relative difficulty can still support comparisons between layouts on the same device. Population means must not hide accessibility needs, which require separate verification.

Applying it

• Test key pointing tasks with target hardware and representative users; do not use desktop-mouse data to justify touch or assistive settings.
• Record hit areas, display scale, input gain, and posture so tests across versions remain comparable.
• Treat model predictions as design hypotheses, then validate with actual time, misses, and user distributions instead of accepting imported coefficients as a gate.

Related

• Same group: B1.11.1 The law assumes rapid aimed movement; slow tracking and dragging are outside its scope · 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 · B1.02 Fitts's law on touchscreens
• Search terms: Fitts's law calibration · throughput · effective width