How do psychologists choose who to study and control the variables in an experiment?
Sampling methods (random, opportunity, systematic, stratified) and variables: independent and dependent variables, operationalisation, extraneous and confounding variables, and controls.
A focused answer to the OCR GCSE Psychology J203 research methods topic on sampling and variables, covering sampling methods (random, opportunity, systematic and stratified), independent and dependent variables, operationalisation, extraneous and confounding variables, and controls.
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What this dot point is asking
OCR wants you to explain sampling methods (random, opportunity, systematic and stratified) and variables: the independent and dependent variables, operationalisation, extraneous and confounding variables, and controls.
Sampling methods
- Random sampling: every member of the population has an equal chance of selection (for example, names drawn from a hat). Strength: likely unbiased and representative. Weakness: time-consuming, and you need a full list of the population.
- Opportunity sampling: using people who are available and willing at the time. Strength: quick, cheap and convenient. Weakness: likely biased and unrepresentative, so it may not generalise.
- Systematic sampling: selecting every nth person (for example, every 5th name on a list). Strength: objective and avoids researcher bias. Weakness: not truly random, and the pattern could coincide with a hidden one.
- Stratified sampling: dividing the population into subgroups (strata, such as age groups) and sampling in proportion to their size. Strength: most representative. Weakness: complex and time-consuming.
Variables
In an experiment:
- The independent variable (IV) is what the researcher changes or manipulates.
- The dependent variable (DV) is what the researcher measures to see the effect.
For example, the vague IV "revising with music" must be operationalised (for example, "20 minutes with instrumental music at a set volume" versus "20 minutes in silence"), and the vague DV "memory" must be operationalised as something measurable ("the number of words correctly recalled from a 20-word list").
Extraneous, confounding variables and controls
- An extraneous variable is any other variable (besides the IV) that could affect the DV (such as noise, time of day, or differences between participants).
- A confounding variable is an extraneous variable that varies systematically with the IV, so it offers an alternative explanation for the results and ruins the experiment.
- Controls are the steps taken to keep extraneous variables constant (for example, testing everyone in the same room, at the same time, with the same instructions), so any change in the DV can be attributed to the IV.
Evaluating sampling and variable control
These choices matter because they determine whether results are valid and generalisable. The strength of careful sampling (random, stratified) is a representative sample that generalises; the weakness is that the most representative methods are the most time-consuming, so researchers often use quicker opportunity samples that may be biased. Tight control of variables raises internal validity (we can trust the IV caused the effect) but, taken to extremes in a lab, can lower ecological validity. Good research balances representativeness, control and realism, linking directly to the method choices in planning research.
Try this
Q1. Name four sampling methods. [4 marks]
- Cue. Random, opportunity, systematic and stratified.
Q2. What does it mean to operationalise the dependent variable? [2 marks]
- Cue. Define it precisely so it can be measured (for example, memory as the number of words recalled).
Q3. What is a confounding variable? [2 marks]
- Cue. An extraneous variable that varies with the IV, offering an alternative explanation for the results.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20204 marksExplain one strength and one weakness of opportunity sampling. (J203/01, Research methods)Show worked answer →
A 4-mark Explain item rewards one developed strength and one developed weakness.
Opportunity sampling means choosing participants who are simply available and willing at the time, such as asking people who happen to be nearby. Strength: it is quick, cheap and convenient, because the researcher uses whoever is to hand, so a sample can be gathered easily and without delay. Weakness: it is likely to be biased and unrepresentative, because the people available in one place at one time may not reflect the wider population (for example, only students in a college corridor), so the results may not generalise.
Markers reward a developed strength (quick, cheap, convenient) and a developed weakness (biased or unrepresentative sample, poor generalisation).
OCR 20214 marksExplain what is meant by operationalising the variables in an experiment, using an example. (J203/01, Research methods)Show worked answer →
A 4-mark Explain item rewards a definition of operationalisation and a clear example.
Operationalising a variable means defining it precisely in terms of how it will be measured or manipulated, so the study can be carried out and repeated. For example, if the independent variable is "revising with music", it must be operationalised as, say, "revising for 20 minutes with instrumental music at a set volume" versus "revising for 20 minutes in silence". The dependent variable "memory" must be operationalised as something measurable, such as "the number of words correctly recalled from a 20-word list". Without operationalisation the variables would be too vague to test.
Markers reward defining operationalisation (making a variable measurable or precisely defined) and a clear example operationalising both an IV and a DV.
Related dot points
- Planning research: aims and hypotheses (directional and non-directional, the null hypothesis), experimental methods (laboratory, field and natural experiments), and experimental designs (independent measures, repeated measures and matched pairs).
A focused answer to the OCR GCSE Psychology J203 research methods topic on planning research, covering aims and hypotheses (directional, non-directional and null), the experimental methods (laboratory, field and natural experiments) and the experimental designs (independent measures, repeated measures and matched pairs).
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A focused answer to the OCR GCSE Psychology J203 research methods topic on data and statistics, covering quantitative and qualitative data, primary and secondary data, the measures of central tendency (mean, median, mode), the range, percentages and ratios, and how to present data in tables, bar charts and scatter graphs.
- Ethical issues and guidelines in psychological research: informed consent, deception, the right to withdraw, protection from harm, confidentiality and debriefing, and how researchers deal with these issues.
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- Non-experimental methods: observations, self-report (questionnaires and interviews), case studies and correlations, plus the concepts of reliability and validity used to evaluate all research.
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Sources & how we know this
- OCR GCSE (9-1) Psychology J203 specification — OCR (2017)