The specification says: Plan and carry out an investigation into the effect of a factor on human reaction time.
Aim
To find the effect of a distraction on the time it takes a person to catch a falling ruler.
Background
Reaction time is the time between a stimulus and the response. In this practical the stimulus is the ruler starting to fall and the response is the hand closing to catch it.
The signal travels along a pathway: a receptor (the eye) detects the stimulus, a sensory neurone carries impulses to the central nervous system, a relay neurone passes the message on, and a motor neurone carries impulses to an effector (the muscles in the hand). The response is voluntary, so the brain is involved.
You cannot easily time a reaction with a stopwatch, because your own reaction time to the stopwatch gets in the way. A falling ruler solves this: the further the ruler falls before it is caught, the longer the reaction time. Conversion from distance to reaction time (the time an object takes to fall that distance, using g = 9.8 m/s²): 10 cm = 0.14 s, 15 cm = 0.17 s, 20 cm = 0.20 s, 25 cm = 0.23 s, 30 cm = 0.25 s. For distances in between, read between the values.
Many factors could affect reaction time, such as caffeine, a distraction, practice, tiredness or which hand is used. You choose one factor to change and keep everything else the same.
Hypothesis
Reaction time will be longer when the volunteer is distracted, because the brain has to deal with another task at the same time, so the ruler will fall further before it is caught.
Variables
| Independent | Whether the volunteer is distracted (counting backwards) or not |
|---|---|
| Dependent | Distance the ruler falls before it is caught (cm), converted to reaction time (s) |
| Control |
|
Equipment
- 30 cm ruler (a 1 m metre rule can also be used)
- Chair and a table with a clear edge
- Pen and results table
- Calculator
- Conversion table for distance to reaction time (given in the background above)
Risk assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Falling ruler | Minor bruise or pinch to fingers | Use a light plastic or wooden ruler and drop it from a low height. Keep fingers clear of the edge of the table. |
| Chairs and trailing bags | Tripping over bags or falling off a chair when lunging for the ruler | Sit with feet flat on the floor and keep the area around the table clear. |
| Human volunteers | A volunteer may be pressured, or may be unwell or allergic if a food or drink is used as a factor | Volunteers must agree freely and may stop at any time. Do not use caffeine drinks or other substances unless your teacher has approved it and consent has been given. |
Method
- Choose the factor to change. In this version the factor is a distraction: the volunteer counts backwards in threes aloud while trying to catch the ruler.
- The volunteer sits with their forearm resting on the table and their dominant hand over the edge, thumb and forefinger about 1–2 cm apart.
- The partner holds the ruler vertically so that the zero mark is level with the top of the volunteer’s thumb.
- The volunteer watches the ruler, ready to catch it.
- Without warning, the partner lets go of the ruler. The volunteer catches it by closing the thumb and finger as fast as possible.
- Read the distance on the ruler at the top of the volunteer’s thumb and record it in cm.
- Repeat 5 times with no distraction. Change the time before each drop each time so that the volunteer cannot predict it. Allow a short rest between drops.
- Now the volunteer counts backwards in threes aloud from 100 while the partner drops the ruler without warning.
- Record the distance and repeat 5 times with the distraction.
- Calculate the mean distance for each condition.
- Use the conversion table to change each mean distance into a reaction time in s.
- Swap roles, or collect class data, so that results from several volunteers can be compared.
Results
Fill this table in as you go. Print the PDF for a copy to write on.
| Condition | Test 1 (cm) | Test 2 (cm) | Test 3 (cm) | Test 4 (cm) | Test 5 (cm) | Mean distance (cm) | Reaction time (s) |
|---|---|---|---|---|---|---|---|
| No distraction | |||||||
| Counting backwards |
Drawing the graph
Bar chart of mean reaction time in s (y-axis, labelled ‘Reaction time (s)’) against condition (x-axis, one bar for ‘No distraction’ and one for ‘Counting backwards’). Bars of equal width with gaps between them, because the independent variable is a category. Use more than half the grid and equal scale steps starting at zero.
Example results and answersPractice data, conclusion, errors and 10 exam questions (26 marks) with mark schemes
Example results
| Condition | Test 1 (cm) | Test 2 (cm) | Test 3 (cm) | Test 4 (cm) | Test 5 (cm) | Mean distance (cm) | Reaction time (s) |
|---|---|---|---|---|---|---|---|
| No distraction | 12 | 14 | 13 | 15 | 11 | 13.0 | 0.16 |
| Counting backwards | 19 | 22 | 20 | 24 | 20 | 21.0 | 0.21 |
Conclusion
The mean distance the ruler fell was 13.0 cm with no distraction and 21.0 cm with the distraction. This is a reaction time of about 0.16 s compared with about 0.21 s. So the distraction increased reaction time, which agrees with the hypothesis. When the volunteer is counting, the brain has to process more than one task, so impulses from the eye to the muscles are dealt with more slowly. The results are from one volunteer only, so they do not show that everyone is affected in the same way.
Errors and improvements
| Error | Effect on the results | Improvement |
|---|---|---|
| The volunteer predicts the drop (a systematic error that makes reaction time look too short). | The volunteer starts to close their fingers before the ruler moves, so the distance is too small. | The partner should drop the ruler without warning, and change the time before each drop. |
| The ruler is not released in the same way each time, or the zero is not level with the thumb. | The distance is read from a different starting point, adding random error to the results. | Check the zero is level with the top of the thumb each time, and use the same person to drop the ruler. |
| Practice: the volunteer gets quicker with each test. | Later tests are quicker whatever the condition, so the effect of the factor is mixed up with the effect of practice. | Give practice goes first, and alternate the order of the conditions. |
| Only one volunteer and only five repeats. | Results may not be typical of other people and anomalies have more effect on the mean. | Test several volunteers, take more repeats and ignore anomalous results when calculating the mean. |
| Reading the ruler to the nearest cm is not very precise at short distances. | A 1 cm error is a large fraction of a short distance. | Use a computer-based reaction timer, which measures to 0.001 s. |
Exam questions
10 questions, 26 marks. Write your answers on paper, then open each mark scheme.
Question 1
State the independent variable in the investigation described in the sample results.
Show mark scheme for question 1
- whether the volunteer was distracted / counting backwards or not (1)
Question 2
Give two variables that the students should control in this investigation.
Show mark scheme for question 2
- same volunteer / same hand (1)
- same ruler / same person dropping the ruler / same height of drop / same starting position of the ruler / same time of day (1) allow any two sensible control variables
Question 3
Explain why the partner drops the ruler without warning.
Show mark scheme for question 3
- so the volunteer cannot predict / anticipate the drop (1)
- so the result is a true measure of the reaction time / the volunteer does not start to close their hand early (1)
Question 4
A volunteer caught the ruler after it had fallen the distances shown in the table. One result is anomalous. Identify the anomalous result and calculate the mean of the other results.
| Test | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Distance (cm) | 22 | 19 | 21 | 35 | 20 | 23 |
Show mark scheme for question 4
- 35 (cm) (1)
- (22 + 19 + 21 + 20 + 23) ÷ 5 (1)
- = 21 (cm) (1)
Question 5
Describe the pathway taken by impulses from the eye to the hand when a person catches a falling ruler. Use words from the box. effector, motor neurone, receptor, relay neurone, sensory neurone, central nervous system
Show mark scheme for question 5
- receptor (in the eye) detects the stimulus (1)
- impulses along a sensory neurone to the central nervous system, where a relay neurone passes them on (1)
- impulses along a motor neurone to the effector (muscle in the hand), which contracts (1)
Question 6
The student displays the mean reaction times for the two conditions on a chart. Name the type of chart that should be used and give a reason.
Show mark scheme for question 6
- bar chart (1)
- because the independent variable is a category / not continuous (1) allow because the conditions are separate groups
Question 7
Use the sample results (mean reaction time 0.16 s with no distraction and 0.21 s when counting backwards) to calculate the percentage increase in reaction time caused by the distraction. Give your answer to 2 significant figures.
Show mark scheme for question 7
- change = 0.21 − 0.16 = 0.05 s (1)
- 0.05 ÷ 0.16 × 100 (1)
- = 31 % (1) an answer of 31.25 or 31.3 (not to 2 significant figures) scores 2 marks
Question 8
Suggest one way to improve the accuracy of measuring reaction time in this investigation.
Show mark scheme for question 8
- use a computer-based / electronic timer (1)
- which measures time directly to a smaller unit / removes the need to read a ruler (1)
- Max 2. allow for 1 mark: repeat with more volunteers and calculate a mean
Question 9
Explain why the results from one volunteer cannot be used to make a conclusion about all students.
Show mark scheme for question 9
- people differ / one person may not be typical (1)
- need a larger sample / several volunteers to show the same trend (1)
Question 10
A student thinks that drinking a caffeine drink will shorten a person’s reaction time. Plan an investigation to test this using the ruler-drop method. Include how you would make the investigation safe and ethical.
Show mark scheme for question 10
| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, logical plan with the independent variable (caffeine drink or a drink without caffeine as a control), the dependent variable (distance the ruler falls, converted to a time), at least three control variables, repeats with a mean, and at least one safety or ethical point. The plan could be followed by another person. |
| 2 | 3–4 | A plan with most key steps. The independent and dependent variables are identified, but control variables, repeats or safety / ethics are weak or missing. |
| 1 | 1–2 | A few simple points, for example ‘drink coffee and catch a ruler’, with no control or little detail. |
Indicative content
- test the volunteer before drinking, then after a caffeine drink and wait for a set time (for example 20 minutes)
- use a control: the same volunteer drinks a drink with no caffeine on another day (or group), ideally without knowing which one it is
- ruler drop with the partner dropping without warning; record distance fallen in cm
- repeat at least five times for each condition and calculate a mean; ignore anomalies
- control volunteer, hand, ruler, drop height, time of day, amount of drink
- safety / ethics: informed consent, check for allergies or health conditions, limit the amount, volunteers can stop at any time, follow teacher approval
Exam tips
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026