Your reaction time is the time between a stimulus and your response. You can measure it with nothing more than a metre ruler:

• The catcher rests their forearm on a table edge with thumb and first finger apart, level with the zero mark of a ruler held vertically by a partner.
• The partner lets go without warning. The catcher grips it as soon as they see it move.
• Read the distance at the top of the thumb. A longer distance means a longer reaction time.
• Repeat several times, use the same hand each time, and take a mean.

Independent variable: whatever you change (for example a drink with caffeine, or practice). Dependent variable: the distance the ruler falls. Control variables: same person, same hand, same ruler, same dropper, no distractions.

The dropper must vary the moment they let go. If the catcher can guess the timing, they are not reacting at all.

The ruler falls freely, speeding up as it goes, so the time is not proportional to the distance. With the distance d in metres and g = 9.8 m/s² (given in the question):

time = √(2 × distance ÷ g)
t = √(2d ÷ g)

1. The ruler falls 14.0 cm before it is caught
2. d = 14.0 ÷ 100 = 0.140 m
3. t = √(2 × 0.140 ÷ 9.8) = √0.02857
4. t = 0.169 s, about 0.17 s (2 s.f.)

This equation comes from physics; in a biology paper it is given to you, or you use a conversion table made from it.

Convert centimetres to metres first, and take the square root last. Forgetting either gives a time several times too big.

Reaction times vary from go to go. Leave out any result that is clearly out of line (an anomaly, often because the catcher was not ready), then take the mean of the rest.

Caffeine is a stimulant: many people react faster after a strong coffee or an energy drink. To compare, use the percentage change:

1. mean before = 260 ms, mean after = 234 ms
2. change = 260 − 234 = 26 ms
3. percentage decrease = 26 ÷ 260 × 100 = 10%

One person tested a few times shows what happened to that person. It does not show the effect on everyone.