Science required practicals
How do you do the reaction time required practical?
The reaction time required practical asks you to plan and carry out an investigation into a factor that affects human reaction time. The usual method is the ruler drop test: a partner drops a ruler without warning and you catch it as fast as you can. How far it fell tells you your reaction time, using a conversion table. You repeat, then compare a condition such as your other hand. It is on GCSE Biology and Combined Science.
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the reaction time practical investigate?
It asks you to plan and carry out your own investigation into something that affects how quickly a person reacts. You choose the factor. Safe choices include which hand you use, or whether there is a distraction such as music.
Catching a falling ruler is a response you control, not a reflex. Your eyes detect the ruler moving, which is the stimulus. A signal travels along sensory neurones to your brain, and your brain sends a signal along motor neurones to the muscles in your hand. The time all of that takes is your reaction time.
It is one of AQA's required practicals in both GCSE Biology and Combined Science: Trilogy, in the homeostasis and response topic.
What equipment do you need?
Very little: a 30 cm ruler and a partner to drop it, plus a table that turns each distance into a time. A computer reaction test can replace the ruler.
- A 30 cm ruler
- A partner to drop it
- A desk or bench, so your arm rests in the same position every time
- A conversion table, to turn the distance the ruler fell into a reaction time
- Whatever your factor needs, such as music for a distraction test
- A computer reaction test, if you use one instead of a ruler
What is the method?
Do the ruler drop test several times under one condition, change the factor, then test again in exactly the same way.
- Sit with your forearm resting on the desk and your hand over the edge.
- Your partner holds the ruler upright, with the zero mark level with the top of your thumb. Keep your thumb and first finger apart, not touching the ruler.
- Without warning, your partner lets go. Catch the ruler as quickly as you can.
- Read the measurement level with the top of your thumb. That is how far the ruler fell.
- Repeat several times, say five, with a different wait before each drop so you cannot guess when it is coming.
- Change the factor you are testing, for example switch to your other hand, and do the same number of catches.
- Convert each distance into a reaction time using the conversion table.
What are the variables?
The factor you choose is the independent variable and reaction time is the dependent one. People get better at catching as they go, so the order of testing matters: do a few practice catches first, or take turns between the two conditions.
| Variable | In this practical |
|---|---|
| Independent | The factor you choose, for example which hand is used |
| Dependent | Reaction time, worked out from how far the ruler falls |
| Control | The same person dropping the ruler, and the same person catching it |
| Control | The starting position: zero mark level with the top of the thumb, arm resting on the desk |
| Control | The same ruler, and the same number of catches for each condition |
How do you work out and present the results?
Work out the mean distance for each condition, leaving out anomalies, then convert it to a time with the table. A shorter distance means a faster reaction.
Worked example: five catches with your writing hand land at 12, 15, 9, 14 and 10 cm. The mean is 60 ÷ 5 = 12 cm, and the range runs from 9 to 15 cm. On a standard conversion table, 12 cm is about 0.16 seconds.
Distance and time are not in proportion. A 5 cm catch is about 0.10 seconds and a 20 cm catch is about 0.20 seconds, so four times the distance is only twice the time. That is why you convert with the table rather than comparing distances as if they were times.
If your factor is a category, such as which hand you used, draw a bar chart of the mean reaction time for each one. If it is a number, such as how many practice catches you have had, draw a line graph.
How do you make it accurate and safe?
Guessing is the biggest threat to accuracy. If the catcher can see a countdown or a twitch of the dropper's hand, the result measures anticipation, not reaction. Vary the wait before each drop, and watch the ruler rather than your partner. A computer reaction test measures in thousandths of a second and takes the dropper out of the test, so its results are more precise.
The practical itself is low risk. The thing to plan for is people: anyone you test should agree to take part and be free to stop at any time. Don't test anything someone eats or drinks, such as caffeine, unless your teacher has agreed to it.
What do exam questions ask about it?
Because this practical asks you to plan, questions often give you an idea to test and ask how to make it a fair test: what to keep the same, and why the catcher must not know when the ruler will drop. Expect to calculate a mean and spot an anomaly, which might be a catch where the catcher guessed the drop and got an unusually short distance. You may need to read a time off a conversion graph, or explain why a computer test gives more precise results. Some questions link back to the nervous system and ask for the path from stimulus to response. The 6-mark version asks you to plan the investigation.
An exam-style question we wrote: "A student's catches got shorter with every attempt. Suggest why, and how the student could stop this affecting their results." A good answer: the student was getting better with practice, so they should do some practice catches before recording any results.
Common mistakes
- Letting the catcher anticipate the drop. No countdowns, and vary the wait.
- Changing who drops the ruler partway through. Keep the same dropper for every catch.
- Treating distance as time. Convert with the table: four times the distance is only about twice the time.
- Calling the catch a reflex. It is a conscious response that goes through your brain.
- Drawing a line graph for a factor that is a category, such as left or right hand. Use a bar chart.