The Human Nervous System » Required Practical: Reaction Time

The aim of the practical

You have seen that reaction time is the time between a stimulus and a response. In this practical you plan and carry out your own investigation to find out whether a chosen factor changes it. The factor might be practice, using your other hand, or a stimulant such as caffeine.

The question might be: does practice change a person's reaction time?

A good hypothesis gives a prediction and a reason: reaction time will get shorter with practice, because the volunteer gets used to the test.

Key terms:

• Conversion table: a table that changes the distance the ruler fell into a time in seconds.
• Anomalous result: a result that does not fit the pattern of the others.

Measuring reaction time with a ruler

The apparatus is simple: a metre rule, a chair and a table. A stopwatch is not good for this job because the person pressing it has a reaction time of their own, which would add error to the results.

The ruler drop test records time without a clock. Instead you measure how far the ruler falls before it is caught. The longer the fall, the slower the reaction.

You then change distance into time using a conversion table, which is usually given to you. A ruler that falls further has been falling for longer. Here is an example.

• 10 cm: 0.14 s
• 20 cm: 0.20 s
• 30 cm: 0.25 s
• 40 cm: 0.29 s

The variables

Keep everything else the same so that only the chosen factor can have caused a change. If practice is the factor, use the same person, the same hand and the same ruler throughout. If the hand is the factor, use the same person and the same ruler. The catcher's thumb and finger must be level with the zero mark on the ruler every time, and the person must not be distracted.

The method

1. Sit the volunteer on a chair with their forearm resting on the edge of the table and their hand over the edge.
2. The helper holds the ruler vertically, with the zero mark level with the top of the volunteer's thumb and first finger, about 1 cm apart.
3. The helper drops the ruler without warning. The volunteer catches it as fast as they can.
4. Read the distance from the zero mark to the top of the thumb, in cm.
5. Repeat at least three times for the same condition, then change the factor (for example, do the same test with the other hand) and repeat again.
6. Use the conversion table to change each distance to a time in seconds.

Repeats, means and anomalies

One drop is not reliable, because a person may be distracted or lucky. Repeating the test and finding a mean makes the results more reliable.

Look at the repeats before you find the mean. A result far from the others is anomalous. Check whether it was caused by a mistake, then leave it out of the mean and say that you did so.

Showing and using the results

Record your results in a table with a column for each repeat, then a column for the mean distance and a column for the time. Give every column a heading with its unit.

You may need to translate between numbers and a graph. If the factor is a category, such as left hand and right hand, draw a bar chart. If the factor is a number, such as attempt number 1 to 5, draw a line graph with the factor on the x-axis and mean reaction time on the y-axis.

Describe the pattern in a sentence, for example: reaction time got shorter with each attempt, so the data supports the hypothesis. Then evaluate the investigation.

• Only one volunteer was used, so the result may not be true for other people.
• The distance was measured to the nearest centimetre, which limits how precise the times are.
• With practice, the volunteer may have started guessing when the ruler would fall.
• More repeats and more volunteers would make the results more reliable.

Working safely and ethically

The spec says that you must use humans safely and ethically when measuring a body response.

• The volunteer must agree to take part and can stop at any time.
• Nobody should be pressured to take part, or be made to feel bad about a slow result.
• Keep the chairs steady and the area clear, and do not drop the ruler from above the face.
• If a drink such as caffeine is used as the factor, your teacher must approve it first. Only people who are allowed it and who choose to take part should be used.
• Keep the results private if the volunteer asks.

Exam-style question

A student investigated whether practice affects reaction time. The same person caught a dropped ruler. The distances the ruler fell were 30 cm, 40 cm and 20 cm without practice.

(a) Calculate the mean distance. [1 mark]

(b) Name the independent variable and the dependent variable. [2 marks]

(c) Give two variables the student should keep the same. [2 marks]

(d) Suggest one way the student could make the results more reliable. [1 mark]