Required practical 6 · Biology · Biology Paper 2, both tiers

Human reaction time

Plan and carry out an investigation into the effect of a factor, such as caffeine or practice, on human reaction time.

The exam trap. A distance is not a time. Practice improves the second set, so use a control or randomise the order.

Variables

Independent (you change): The factor tested, for example drinking caffeine
Dependent (you measure): Reaction time (from the distance the ruler falls, using a conversion table)
Control (you keep the same): The same hand (dominant hand)
The ruler's starting position, with zero at the top of the thumb
The same person dropping the ruler, with no warning
No distractions
The same time of day

Sit with your forearm on the table and your hand over the edge, thumb and finger apart.
Why A fixed arm position keeps the test the same each time.
A partner holds the ruler with zero level with the top of your thumb and drops it without warning.
Why Starting at zero means the reading is the distance fallen.
Catch the ruler as fast as possible and read the distance at the top of your thumb.
Why The distance fallen depends on the reaction time.
Repeat several times and calculate the mean.
Why Reaction times vary; a mean reduces random error.
Change the factor (for example, after a caffeinated drink and waiting 10 minutes) and repeat.
Why This tests the effect of the independent variable.
Convert each distance to a time using the conversion table.
Why Distance is not a time: the ruler accelerates as it falls.

Hazard	How to reduce the risk
Caffeine can affect some people.	Only use caffeine if safe for that person; water is a control.
A falling ruler could hit the face.	Keep the face away from the ruler.

Condition	Mean distance in cm	Reaction time in s
Before caffeine	19	0.20
After caffeine	14	0.17

A shorter distance means a shorter reaction time.
Doing the second set after the first introduces a practice effect: use a control group or a decaffeinated drink.
One person's results cannot be generalised.

Treating the distance as if it were a time.
Instead Use the conversion table, or a timer or app.
Naming both the factor and the distance as the independent variable.
Instead The independent variable is only the factor you change.
Ignoring the practice effect when the second test follows the first.
Instead Randomise the order or use separate people and a control drink.