Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Measuring reaction time of a human nerve-controlled reaction

This is likely to be a very familiar protocol. It is simple enough to be used as the basis for a planning exercise (where planning is the main learning objective). Run through and point out features that need to be controlled – in the core experiment, and also issues about ‘double-blind’ testing.

Lesson organisation

Demonstrate the core technique and ask students to devise the detail – to ensure that as many control variables as possible are covered and a suitable number of repeats. Collect data for the whole group. Explain that it would be interesting to find out whether the reaction time is influenced by caffeine in our bodies. Set up a double-blind test for the effect of caffeine.

Apparatus and Chemicals

For each group of students:

Metre ruler

For the class – set up by technician/ teacher:

Caffeinated and de-caffeinated drinks (eg cola or coffee) – fixed volumes with known amounts of caffeine (Note 1), in identical cups, labelled for a ‘double-blind’ test (Note 2)

Health & Safety and Technical notes

Take care moving around the room with metre rulers – to avoid hitting one another with them, carry them vertically and close to you at all times.

If using hot coffee as your source of caffeine, take reasonable care as with any hot liquid.

Carry out the part of the experiment involving consumption of drinks in a classroom or dining room, rather than a laboratory where chemicals or biomaterials have been used.

Ensure that doses of caffeine are similar to those normally taken and that students are briefed (using the Student briefing and consent form) to avoid consuming large quantities of caffeine before or after the investigation.

1 CLEAPSS Laboratory Handbook, section 11.8.1 states that an average cup of coffee contains around 100 mg of caffeine. Cola drinks contain less: between 35–50 mg per 330 ml can or 12 ounce bottle. A 250 ml (8.3 oz) can of Red Bull contains 80 mg of caffeine, 1000 mg of taurine, 600 mg of glucuronolactone (a precursor of taurine) and 27 g of sugar (unless the sugar-free version is consumed). Some individuals do not tolerate caffeine very well. There are also reports linking Red Bull with aspects of ill health including irregular heart beats, nausea, stomach pains and insomnia, although such effects arose after consuming far greater amounts than would be drunk in a school investigation. In an investigation of reaction times, students should consume caffeine in a drink (cola, coffee etc) they normally take, already being aware of any possible adverse effects. Teachers should limit the quantity of caffeine that pupils consume so that the total intake in the investigation is no more than a single cup or can. To achieve this, students should have been instructed, in advance, not to have consumed a caffeine-containing drink before the lesson. This would also be required to avoid compromising the investigation. Students should also be told to avoid further caffeine-containing drinks until much later in the day.

2 Double blind test: Label each cup with an apparently complicated code (a letter and 4 numbers perhaps) and keep records in the prep room, or with the teacher, showing which codes have caffeine and which do not. If using soft drinks, the drinks used should contain known amounts of caffeine and similar amounts of sugar or artificial sweetener, and similar complements of colouring and other ingredients. If using coffee, it can be difficult to estimate the exact concentration of caffeine, but you could compare the effect of different concentrations of coffee. Where the caffeine concentrations are not equivalent, another variable is introduced that could be responsible for any change in reaction time.

Ethical issues

Students are effectively test subjects for this practical. They should sign the attached form to state that they understand the procedure, agree to be involved, and have the right to stop their participation and remove their results at any time. Whereas giving pupils measured doses of caffeine in the form of ProPlus or other tablets could be seen as questionable, consuming caffeine in cups of coffee or in soft drinks is socially acceptable for most people and much less likely to stimulate adverse comment.


SAFETY: Take care carrying metre rulers and take care with hot coffee. Do not consume drinks in a laboratory where chemicals or biomaterials are regularly used. Do not consume extra caffeinated drinks before or after the practical.


a Demonstrate the basic procedure for the investigation. Ask one student to sit (or stand) where all can see what is going on. Ask the student to hold the metre ruler vertically in one hand so that their hand is a little over 1 m from the floor. Then take hold of the ruler and ask them to let go, lift the ruler until it is mostly above the student’s hand. Tell them that they have to catch the ruler when you drop it. Drop it and then read the distance dropped (in cm) from the ruler. You could repeat the process introducing deliberate inconsistencies – turning the ruler over (so the zero point is at the opposite end), starting with the ruler higher or lower relative to the student’s hand, warning the student that you are about to drop the ruler (with a 1-2-3 countdown or ‘ready, steady, catch’), holding the ruler at an angle, asking them to look away or wear a blindfold, starting with the ruler not in contact with the student’s hand, or using a different student.


b Ask your students to describe in detail how to carry out this test to control as many variables as possible. They could work in pairs, then share common decisions to develop a group description, and then collate all the factors they decide to control on the board. The student sheet for this practical is a proforma with space to list the factors they have decided to control.

c Deciding on the best number of repeats: If you have demonstrated a single test with each of two students, you can then discuss the need to repeat the test several times and take a mean average in order to get reliable results. The spreadsheet Number of repeats effect on mean average (17 KB) shows the effect of taking several results and then calculating a mean average.

d Discuss with students the idea that caffeine might improve our reaction time. Ask them to plan how they could test this idea. Evaluate the plans. Introduce the idea of a double-blind test if necessary, and include that in the plans.

e Carry out a ‘double-blind’ test where you provide fixed volumes of drink with known amounts of caffeine. The drinks should contain similar amounts of sugar (or artificial sweetener) and similar complements of colouring and other ingredients. Test reactions before taking the drinks and about 10 minutes later. Compare the mean averages again.

Teaching notes

Deciding on a number of repeats
The spreadsheet shows the effect of taking several results and then calculating a mean average. It shows that above a certain number of repeats, taking more readings makes less and less difference to the final result. In an investigation like this, three to five repeats are likely to be adequate. The spreadsheet includes data encompassing a narrow range – but you could change the figures in the second column to see the effect of an outrageous anomaly, or a small one, or to track the data you are collecting as it is collected.

Double-blind test
The essence of a double-blind test is that neither the test subjects nor the investigators in contact with the test subjects should know whether the test substance or a blank is being administered. In this way, the investigators cannot subconsciously influence the test subject; there should not be an observer bias in their results (Note 2).

To evaluate students’ ideas about the double-blind test with caffeine, ask questions such as:

Q: How many people should we test with caffeine? A: More than one. As many as possible to eliminate other differences from person to person.

Q: How long after taking caffeine should we test them? A: Long enough for the caffeine in a drink to have been absorbed. Ten minutes or so.

Q: Should we tell them that we are testing them with caffeine? If so, why and if not, why not? A: If we tell them, they may react differently because they think the drink ought to have an effect. If we don’t, they don’t have the chance to warn us if they are allergic or sensitive to caffeine, or if they do not take stimulants for other reasons.

Q: How will we be sure that any effect we see is due to caffeine and not to something else? A: By controlling as many variables as possible. But if we use manufactured soft drinks which we can be sure are safe to drink, there may be variations in ingredients that we cannot eliminate.

Converting distance to time
Objects fall at a calculable speed due to the effect of gravity. Therefore, you can convert the distance the ruler has dropped into a time measurement. This might make a more meaningful, or more interesting comparison. Attached here Reaction timer - distance to time converter (318 KB) is a ruler marked in seconds as well as centimetres. If you copy it to card or paper and stick it to a ruler, joining the strips end to end exactly on the cut marks, you can read time directly from each drop. Or you could use it as a visual resource and convert the distances dropped to seconds.

The significance of longer reaction times
Attached here Speed of reaction tester (215 KB) is an adaptation of part of a card for testing reaction times produced by ROSPA (Royal Society for the Prevention of Accidents). See the link below for more information about current versions of this card and how to get hold of them from the GEM Road Safety Charity.

Health & Safety checked, May 2009


Download the student sheet Measuring reaction time of a human nerve-controlled reaction (58KB) with questions and answers.
Download the Student briefing sheet and consent form.
Download the spreadsheet Number Of Repeats Effect On Mean Average (17 KB).
Download the distance to time converter Reaction timer - distance to time converter (318 KB)
Download the speed of reaction tester Speed of reaction tester (215 KB)

Related experiments

Observing the effects of exercise on the human body

Assessing skin sensitivity – locating different receptors

Assessing skin sensitivity – touch discrimination

Assessing skin sensitivity – temperature receptors

Web links
In the BBC’s Human body section of their website is an exploration of the effects of sleep on our bodies, including our reaction times. This is illustrated by this game called ‘Sheep Dash!’ which provides an entertaining alternative measure of reaction time if you don’t want to drop rulers!
The GEM Road Safety Charity provides reaction timer cards free of charge. Teams of two players can use card to indicate how fast their partner's reaction is. It is designed to make people stop and think and to promote awareness of stopping distances and the ‘2-second rule’. Although intended for drivers, it could still be useful with younger students.

(Websites accessed October 2011)