How do psychologists choose participants and control variables?
Sampling methods (random, opportunity, stratified, systematic) and variables (independent, dependent and extraneous variables), and how variables are controlled.
A focused answer to Edexcel GCSE Psychology Topic 11, covering sampling methods (random, opportunity, stratified, systematic) and variables (independent, dependent, extraneous) and how they are controlled.
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What this dot point is asking
Edexcel wants you to explain sampling methods (how participants are selected) and variables (the independent, dependent and extraneous variables) and how variables are controlled. These are core research-methods skills tested across both papers, often by asking you to identify variables in a described study or evaluate a sampling method.
Sampling methods
The main methods are:
- Random sampling: every member of the population has an equal chance of selection (for example drawing names). Strength: likely to be representative and unbiased. Weakness: hard to organise and you need a full list of the population.
- Opportunity sampling: using whoever is available and willing at the time. Strength: quick, easy and cheap. Weakness: likely unrepresentative and biased, so poor generalisation.
- Stratified sampling: dividing the population into subgroups (strata) and selecting in proportion to their size. Strength: representative of the population's makeup. Weakness: time-consuming and complex to set up.
- Systematic sampling: selecting every nth person from a list (for example every 10th). Strength: simple and avoids researcher bias in choosing. Weakness: not truly random and the list order could create bias.
A representative sample matters because it allows the findings to generalise to the wider population.
Variables
To get a fair test, researchers control extraneous variables, for example by keeping the test the same in both conditions, standardising procedures and instructions, and using random allocation to balance participant differences. Good control means any change in the DV is more likely to be caused by the IV.
Try this
Q1. Which sampling method uses whoever is available at the time? [1 mark]
- Cue. Opportunity sampling.
Q2. What is the dependent variable? [1 mark]
- Cue. The variable that is measured to see the effect of the independent variable.
Q3. Explain one strength of random sampling. [2 marks]
- Cue. Everyone has an equal chance of selection, so the sample is likely to be representative and unbiased.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20184 marksExplain one strength and one weakness of using an opportunity sample. (Paper 2)Show worked answer →
A 4-mark Explain item rewards a developed strength and weakness of opportunity sampling.
An opportunity sample uses whoever is available and willing at the time. Strength: it is quick, easy and cheap to obtain, because the researcher simply asks people who are nearby, so it saves time and resources. Weakness: it is likely to be unrepresentative and biased, because the people available in one place at one time (for example students on a campus) may not reflect the wider population, so the results may not generalise to others.
Markers reward the explained strength (quick, easy, convenient) and the explained weakness (unrepresentative or biased, so poor generalisation), each developed rather than just named.
Edexcel 20214 marksA psychologist tests whether background noise affects test scores. Identify the independent and dependent variables and one extraneous variable that should be controlled. (Paper 2)Show worked answer →
A 4-mark item rewards correctly identifying each variable and a sensible extraneous variable.
The independent variable (IV) is the presence or level of background noise (for example noise versus quiet), because that is what the researcher changes. The dependent variable (DV) is the test score, because that is what is measured to see the effect. An extraneous variable that should be controlled is the difficulty of the test (it should be the same for both conditions); other examples include the time of day, the lighting, or differences in participants' ability, which should be kept constant or controlled so they do not affect the DV.
Markers reward the IV (background noise, the manipulated variable), the DV (test score, the measured variable), and a relevant extraneous variable to control, with a brief reason.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Psychology (1PS0) specification — Pearson Edexcel (2017)