Chapter 13: Timing & Optimization (.65 Rule)
The scan rate is the most important timing parameter. A research-based starting point is the .65 rule:
Scan Rate ≈ Mean Reaction Time ÷ 0.65
If press times are highly variable, increase scan rate or add acceptance time.
Acceptance Time & Selection Confirmation
Acceptance time adds a delay before confirming a press, reducing accidental triggers.
- Short (150–300ms): advanced users
- Medium (300–600ms): default
- Long (600–1000ms): tremor control
Debounce & Post-Acceptance
Mechanical switches can “bounce,” creating rapid false presses. Debounce ignores those quick spikes, while a short post‑acceptance window prevents accidental double selections.
Timing Guidelines by User Profile
| User Profile | Recommended Rate | Reason |
|---|---|---|
| Beginner | 1500–2000ms | Learning curve, building confidence |
| Intermediate | 1000–1500ms | Balance of speed and accuracy |
| Advanced | 500–1000ms | Maximum efficiency |
| Motor impairment (tremors) | 1500ms + 500ms acceptance | Filter tremors |
| Visual impairment | 1000–1500ms + audio | Time for feedback |
| Cognitive impairment | 1500–2000ms | More thinking time |
| Fatigue (MND/ALS) | Morning 1000ms / Afternoon 1500ms | Energy changes |
Practical takeaway: Use the .65 rule to get into the right neighborhood, then adjust based on accuracy, fatigue, and user preference.
Reaction Time Quick Test
Trial: 0 / 3
Last Reaction Time: —
Average Reaction Time: —
Recommended Scan Rate (.65 rule): —
Scan Time Estimator
Estimated Avg Presses: —
Estimated Time per Selection: —