How do psychologists select participants using sampling methods?
Sampling methods: the target population and sample, random, opportunity, systematic and stratified sampling, and their strengths and weaknesses for representativeness.
A focused answer to AQA GCSE Psychology 3.4, covering the target population and sample, random, opportunity, systematic and stratified sampling, and their strengths and weaknesses for representativeness.
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What this dot point is asking
AQA wants you to define the target population and sample, describe random, opportunity, systematic and stratified sampling, and evaluate each for how representative it is. This is part of research methods and is applied to scenarios, so you need to recognise which method has been used and judge its bias.
Population, sample and representativeness
The four sampling methods
Choosing a method
The trade-off is usually between representativeness and practicality. Stratified sampling is most representative but slow; opportunity sampling is fast but biased; random and systematic sampling sit in between. The right choice depends on the study, but more representative samples allow more confident generalisation to the target population.
Try this
Q1. Define a target population. [1 mark]
- Cue. The whole group the researcher wants to study and generalise to.
Q2. Identify one weakness of opportunity sampling. [1 mark]
- Cue. It is often biased because it uses whoever happens to be available.
Q3. Explain why stratified sampling is usually more representative than opportunity sampling. [2 marks]
- Cue. It selects from subgroups in proportion to the population, so the sample reflects the population's make-up.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksExplain how a researcher could obtain a random sample, and give one strength and one weakness of this method. (Paper 1, Section D)Show worked answer →
A 4-mark item that rewards the method plus one strength and one weakness.
Method: a random sample is obtained by giving every member of the target population an equal chance of being selected, for example by listing everyone and using a random number generator or drawing names from a hat to choose the sample. Strength: because selection is unbiased, the sample is more likely to be representative of the population, so results can be generalised. Weakness: it can be time-consuming and impractical to list and contact every member of a large population, and the random sample chosen could still, by chance, be unrepresentative.
Markers reward an accurate procedure (equal chance for all, named selection method), one developed strength (reduced bias, representativeness) and one developed weakness (impractical or chance bias).
AQA 20223 marksExplain how a stratified sample differs from an opportunity sample. (Paper 1, Section D)Show worked answer →
A 3-mark Explain item rewards a description of each plus the contrast.
A stratified sample divides the target population into subgroups (strata, such as age groups or sexes) and then selects participants from each subgroup in proportion to their size in the population, so the sample reflects the make-up of the population. An opportunity sample simply uses whoever is available and willing at the time. The difference is that stratified sampling deliberately ensures representativeness of key subgroups, whereas opportunity sampling takes whoever is convenient and is therefore more likely to be biased.
Markers reward describing both methods and stating the key difference (planned proportional representation versus using whoever is available).
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Sources & how we know this
- AQA GCSE Psychology (8182) specification — AQA (2017)