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You could carry out a number of investigations to determine the effect of a specific factor on human reaction times.

A simple method to measure the effect is to use the ruler drop test.

Ruler drop test

1. Work with a partner.
2. Person A holds out their hand with a gap between their thumb and first finger.
3. Person B holds the ruler with the zero at the top of person A's thumb.
4. Person B drops the ruler without telling Person A and Person A must catch it.
5. The distance on the ruler level with the top of person A's thumb is recorded in a suitable table.
6. Repeat this ten times.
7. Swap places, and record another ten attempts.
8. You can use the conversion table to help convert your ruler measurements into reaction time or just record the catch distance in cm.

Example results

You could carry out the experiment with and without background noise, or to investigate the effect of a drink containing caffeine on reaction time.