👥 Participant variables
These are differences between the people taking part, such as age, ability, mood, tiredness or past experience. If one group happens to be older or cleverer, that could change the DV.
Experimental Method » Extraneous Variables and Controls
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 7.1.19, 7.1.20, 7.1.21
- What an extraneous variable is and how it can spoil an experiment
- Participant and situational extraneous variables
- What a control is and the main ways to control extraneous variables
- How to spot extraneous variables and suggest controls in a new scenario
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Unlock This CourseWhat is an extraneous variable?
In Independent and Dependent Variables you learned that the researcher changes the independent variable (IV) and measures the dependent variable (DV). But the IV is not the only thing that can change the DV. Other things can sneak in and affect the results too. These are called extraneous variables.
Imagine a researcher tests whether a 10-minute walk helps pupils do better in a spelling test. One group walks before the test and another group does not. The walkers are tested in a hot, stuffy room on a Friday afternoon, and the other group is tested in a cool room on Monday morning. If the walkers score lower, is it because of the walk? Or because they were hot and tired? The researcher cannot tell. The room and the day are extraneous variables.
Key terms:
- Extraneous variable: a variable, other than the independent variable, that can have an impact on the dependent variable in an experiment.
- Control: a step taken by the researcher to reduce, or keep constant, the impact of extraneous variables.
Learn this
An extraneous variable is anything except the IV that could change the DV. We want only the IV to change the DV, so we try to stop extraneous variables getting in the way.
Two kinds of extraneous variable
It helps to sort extraneous variables into two groups. Ask yourself: is this about the people or about the situation?
🏡 Situational variables
These are things about the setting or the way the study is run, such as noise, temperature, time of day, the room, or the instructions given to participants.
Worked example
A researcher tests whether a short warm-up game makes pupils faster at a maths quiz. Pupils in the warm-up group take the quiz in the morning. Pupils in the no-warm-up group take it after lunch. The researcher also lets the teacher in each room explain the quiz in their own words.
IV: warm-up game or not. DV: time taken to finish the quiz.
Extraneous variables: time of day (situational), different instructions from each teacher (situational), and the pupils' maths ability, which may differ between the groups (participant).
Why do extraneous variables matter?
If extraneous variables are not dealt with, the researcher cannot be sure that the IV caused the change in the DV. The results might be down to the noise in the room or to a group of very able pupils instead. The study then does not truly test what it set out to test. This is linked to validity, which you will study in full in a later lesson on validity.
What is a control?
A control is any step a researcher takes to reduce the effect of an extraneous variable or to keep it the same for everyone. The aim is a fair test, where the only important difference between conditions is the IV. Note that a control is a step the researcher takes. It is not the same as a control group, which you met in Experimental Conditions and Control Groups.
Some common controls:
- Standardised instructions: every participant gets exactly the same instructions, often read from the same script. This controls situational differences in how the task is explained.
- Same room and same time: testing everyone in the same place, at the same time of day, with the same temperature and noise level. This keeps situational variables constant.
- Random allocation: sharing participants out to conditions by chance, so that differences such as age or ability are spread evenly. This is a control for participant variables.
- Counterbalancing: changing the order of conditions across participants in a repeated measures design, so that order is not an extraneous variable.
Worked example
Back to the maths quiz. The researcher could fix the problems like this:
- Time of day: test every pupil in the same morning lesson, in the same room (same time and room).
- Different teacher wording: read the same written script to every pupil (standardised instructions).
- Differences in ability: share pupils between the two conditions by drawing names out of a bag (random allocation).
Now the extraneous variables are controlled, and a difference in quiz times is much more likely to be caused by the warm-up game.
Applying it to a new scenario
In the exam you will be given a study you have never seen. A good method is: 1) find the IV and DV, 2) look for anything else that could affect the DV, 3) say whether it is about the people or the situation, 4) suggest a control that fits.
Worked example
A researcher wants to know if drinking a glass of orange juice before a reaction-time test makes teenagers quicker to press a button. Teenagers who drink juice are tested in the school hall at 9 am. Teenagers who drink nothing are tested in a noisy corridor at 2 pm. A few of the juice drinkers play computer games every day.
Extraneous variables: noise in the corridor, time of day, and experience of computer games.
Controls: test everyone in the same quiet room at the same time, and randomly allocate teenagers to the juice or no-drink condition so gamers are spread across both groups.
Common mistakes
Common mistakes
- Saying the IV is an extraneous variable. It is not. The IV is the thing we want to affect the DV. Extraneous variables are the other things.
- Being vague. "Other things might change it" earns no marks. Say exactly what, for example "the room was noisy for one group".
- Suggesting a control that does not match the problem. Standardised instructions will not fix a noisy room. Match the control to the variable.
- Mixing up a control and a control group. A control is a step to keep things fair. A control group is a group of participants.
Exam-style question
A researcher wants to find out whether reading a funny story makes children feel happier than reading a factual article. Twenty children read the funny story in a warm sports hall on a Monday and rate their happiness from 1 to 10. A different twenty children read the article in a cold storeroom on a Friday and give a happiness rating. A different researcher runs each group and explains the task in their own words.
(a) Identify two extraneous variables in this study. [2 marks]
(b) For one of your extraneous variables, suggest a control the researcher could use. [2 marks]
Model answer
(a) The room was different: a warm sports hall compared with a cold storeroom (1). Any one other from: the day was different (Monday compared with Friday), or the instructions were different because each researcher explained the task in their own words (1).
(b) For the room: test both groups in the same room at the same temperature (1) so the room cannot affect the happiness ratings (1). Any one matching control scores, for example giving every child standardised instructions read from a script (1) so the explanation is identical (1).
Exam tip
In a "suggest a control" question, name the extraneous variable first and then say how your control keeps it the same or reduces its effect. A control that does not link to the variable gets no marks.