Why Your Reaction Time Varies Every Attempt
A single result is noise. The signal lives in the average.
TL;DR — Individual attempts vary by 40–80 ms because attention, breath cycle, posture and random neural firing all change in the seconds between cues. The reliable number is the average of at least five attempts after discarding the slowest.
A 30 ms swing between two consecutive attempts is normal. A 200 ms swing means something interrupted you — usually a thought, a breath in or a tiny shift in eye position.
The biological sources of variance
- Attention oscillates on a roughly 200–300 ms rhythm tied to the brain's alpha waves.
- Breath cycle changes blood oxygen by 1–2% within each inhale-exhale loop.
- Microsaccades — tiny involuntary eye movements — happen every 300–800 ms and briefly blur the cue.
- Neural firing has a stochastic component; even identical conditions produce different latencies.
The hardware sources of variance
Browser timing is precise to about 1 ms, but display refresh adds quantised jitter. A 60 Hz monitor can only show a new pixel every 16.7 ms, so the same neural response gets rounded into different visible buckets. A 144 Hz display reduces that jitter to 7 ms; a 240 Hz display to 4 ms.
How to get a clean number
- Do at least 5 attempts (10 is better).
- Discard the single slowest result — almost always a missed cue.
- Discard the single fastest result if it is more than 50 ms below your next best — likely anticipation.
- Average the rest.
Test under one consistent state
If you want to compare two readings on different days, do them at the same time of day, after the same amount of caffeine, in the same posture. Otherwise you are measuring lifestyle, not reaction time.
Frequently Asked Questions
Why was one attempt 80 ms faster than the rest?
Either you anticipated the cue or the cue happened to land at a peak of your alpha-wave attention cycle. Either way, do not count it as your true number.
How many attempts before the average stabilises?
Seven to ten. After that, additional attempts move the average by less than 5 ms.
Does fatigue widen the variance?
Yes, dramatically. A tired person can swing 150 ms between best and worst attempt on the same test.
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