Investigating Factors Affecting Reaction Time

What does a reaction-time test measure?

Reaction time is the interval between a stimulus and a response. In a ruler-drop test, seeing the ruler begin to fall is the stimulus; closing the thumb and first finger around it is the response. Receptors in the eyes detect the movement, and the nervous system coordinates the hand muscles that catch the ruler. The test measures the time taken for this whole response, not just the time taken for an electrical impulse to travel along one neurone.

You do not need a stopwatch to measure such a short interval. The longer the ruler falls before it is caught, the longer the response took. A smaller catch distance therefore means a faster reaction. A conversion table turns the distance in centimetres into reaction time in seconds: for example, a catch distance of 33 cm corresponds to about 0.26 s. The practical is used to find out whether changing one factor affects this time.

Planning a fair comparison

First choose a factor to change, such as background noise, and state a testable prediction. For example, you might predict that a person will take longer to catch the ruler while music is playing than in quiet conditions. The factor you change is the independent variable. The reaction time you measure is the dependent variable. This is a prediction to test, not a result you can assume in advance.

Measure each person's reaction time before making the change and after making it. Keep other conditions as similar as possible: use the same ruler with its 0 cm end down, start it at the same position, and have each person use their dominant hand each time. Do not give a countdown that lets the catcher anticipate the drop. If background noise is the chosen factor, keep the first set of tests quiet and change the noise for the second set. Any additional equipment depends on the factor chosen. The test should be safe and should not pressure the person taking part.

Carrying out the ruler-drop test

Work with a partner, a metre ruler, a chair and a table. The catcher sits upright, rests the forearm of their dominant arm across the table and lets their hand overhang the edge. The partner holds the ruler vertically, with its 0 cm end between the catcher's thumb and first finger, and tells them to prepare. The catcher looks across the room, then closes their thumb and finger as quickly as possible when the ruler is released. Read the mark level with the top of the catcher's thumb and record the distance in cm.

Allow a short rest and repeat the drop several times. Then swap roles so both partners have a set of results. Make the planned change and repeat the tests for each person in the same way. Repeats help you see whether an apparent difference is consistent rather than due to one unusual catch.

Turning measurements into evidence

Record each attempt in a table. The AQA practical layout has separate distance columns, in cm, for Person 1 before, Person 2 before, Person 1 after and Person 2 after. It then has four matching columns for the converted reaction times, in seconds. Keeping the two people and the before-and-after conditions distinct makes the comparison clear. Use a conversion table to find the reaction time for each ruler reading; do not treat the number of centimetres as though it were already a time.

Reaction-time information can also be shown graphically. Put attempt number on the horizontal axis and reaction time in seconds on the vertical axis, with separate labelled sets of points for before and after. The plot below shows how a table of repeated times can be translated into a graph.

Data for Illustrative reaction times before and after a change

To judge the effect, compare the pattern or mean reaction time before and after for each person. A lower mean time indicates a faster average response. Check any result far from the others before using it to support a conclusion: it could reflect a missed catch, an anticipated drop or an inaccurate reading. Two partners need not have similar reaction times, so a difference between people does not by itself show that the chosen factor caused a change. State whether the results support your prediction and consider how inconsistencies in releasing or reading the ruler might have affected them.