How do psychologists run experiments, define variables and choose an experimental design?
The experimental method: types of experiment (laboratory, field, natural, quasi), independent and dependent variables and operationalisation, hypotheses, extraneous and confounding variables and controls, and experimental designs (independent groups, repeated measures, matched pairs).
An Eduqas A-Level Psychology answer to the experimental method in Component 2. Covers laboratory, field, natural and quasi experiments, independent and dependent variables, operationalisation, hypotheses, extraneous and confounding variables, controls, and the three experimental designs with their strengths and weaknesses.
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What this dot point is asking
The experimental method is central to Component 2. You must know the types of experiment, how to define and operationalise variables, how to write hypotheses, how to control extraneous and confounding variables, and the three experimental designs with their strengths and weaknesses.
The answer
Types of experiment
Variables, operationalisation and hypotheses
Controls and confounds
An extraneous variable is any variable other than the IV that could affect the DV; if it varies systematically with the IV it becomes a confounding variable and ruins the study. Controls include standardisation (the same procedure for everyone), randomisation, and removing or holding constant other influences.
Experimental designs
- Independent groups. Different participants in each condition. No order effects; but participant variables differ and more participants are needed.
- Repeated measures. The same participants in all conditions. Controls participant variables and needs fewer people; but order effects occur, controlled by counterbalancing.
- Matched pairs. Different but matched participants (on key variables). Reduces participant variables without order effects; but matching is difficult and time-consuming.
Examples in context
Example 1. Why a quasi-experiment cannot prove cause. If the IV is an existing difference (for example schizophrenic versus non-schizophrenic, as in Raine), participants are not randomly allocated, so confounds differ between groups and cause and effect cannot be established. This is why Raine's findings are correlational.
Example 2. Counterbalancing in repeated measures. If everyone did the music condition first, practice could inflate the silence scores. Counterbalancing (half do music first, half do silence first) spreads order effects evenly across conditions, showing how design controls a confound.
Try this
Q1. Distinguish between a laboratory and a field experiment. [2 marks]
- Cue. A laboratory experiment manipulates the IV in a controlled setting (high control, artificial); a field experiment manipulates it in a natural setting (more realistic, less control).
Q2. Write an operationalised dependent variable for a study on stress. [2 marks]
- Cue. Stress measured as heart rate in beats per minute (or a score out of 40 on a standardised stress questionnaire).
Q3. Explain one weakness of an independent groups design. [2 marks]
- Cue. Participant variables differ between the two groups and may confound the results, and more participants are needed than for repeated measures.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksA researcher tests whether caffeine improves reaction time. Write a suitable directional hypothesis and identify the independent and dependent variables. [6 marks]Show worked answer →
An application item testing hypotheses and variables (AO2).
Directional (one-tailed) hypothesis: "Participants who consume caffeine will have significantly faster reaction times (lower reaction time in milliseconds) than participants who do not consume caffeine."
Independent variable (IV): whether the participant consumes caffeine (caffeine versus no caffeine), operationalised (for example a 200 mg caffeine drink versus a placebo).
Dependent variable (DV): reaction time, operationalised as the time in milliseconds to press a button when a stimulus appears.
Markers reward a properly operationalised, directional hypothesis stating the predicted direction, and correctly identified, operationalised IV and DV.
Eduqas 20218 marksExplain one strength and one weakness of an independent groups design and of a repeated measures design. [8 marks]Show worked answer →
A knowledge item on experimental designs (AO1/AO3).
Independent groups (different participants in each condition): strength, no order effects (no practice or fatigue from doing both conditions) and the same task can be used; weakness, participant variables differ between groups and may confound the results, and it needs more participants.
Repeated measures (same participants in both conditions): strength, participant variables are controlled because the same people do both conditions, and fewer participants are needed; weakness, order effects (practice or fatigue) can occur, addressed by counterbalancing, and demand characteristics are more likely.
Markers reward an accurate strength and weakness for each design, ideally noting counterbalancing as the control for order effects.
Related dot points
- Non-experimental methods: observation (naturalistic, controlled, participant, non-participant, overt, covert; behavioural categories and sampling) and self-report (questionnaires and interviews; open and closed questions; designing good questions).
An Eduqas A-Level Psychology answer to observation and self-report methods in Component 2. Covers types of observation, behavioural categories, event and time sampling, questionnaires and interviews, open and closed questions, and the strengths and weaknesses of each method.
- Sampling (target population, sample, random, opportunity, volunteer, systematic and stratified sampling; bias and generalisability) and ethics (the BPS principles: informed consent, deception, right to withdraw, protection from harm, confidentiality, and dealing with ethical issues).
An Eduqas A-Level Psychology answer to sampling and ethics in Component 2. Covers target populations and samples, random, opportunity, volunteer, systematic and stratified sampling, sampling bias and generalisability, and the BPS ethical principles with ways of dealing with ethical issues.
- Reliability (internal and external; test-retest, inter-observer; how to assess and improve it) and validity (internal and external; face, concurrent, ecological, temporal and population validity; demand characteristics and investigator effects; how to assess and improve it).
An Eduqas A-Level Psychology answer to reliability and validity in Component 2. Covers internal and external reliability, test-retest and inter-observer reliability, internal and external validity, face, concurrent, ecological, temporal and population validity, demand characteristics and investigator effects, and how to assess and improve each.
- Descriptive statistics: measures of central tendency (mean, median, mode), measures of dispersion (range, standard deviation), levels of measurement (nominal, ordinal, interval), percentages and ratios, and presenting data (tables, bar charts, histograms, scattergrams).
An Eduqas A-Level Psychology answer to descriptive statistics in Component 2. Covers the mean, median and mode, range and standard deviation, levels of measurement, percentages and ratios, and how to present quantitative data in tables, bar charts, histograms and scattergrams, with worked calculations.
- Inferential statistics: probability and significance (), the null and alternative hypotheses, choosing the correct test (the binomial sign test, Mann-Whitney U, Wilcoxon, Spearman's rho, chi-square) from design and level of measurement, observed versus critical values, and Type I and Type II errors.
An Eduqas A-Level Psychology answer to inferential statistics in Component 2. Covers probability and the 0.05 significance level, the null hypothesis, how to choose between the binomial sign test, Mann-Whitney U, Wilcoxon, Spearman's rho and chi-square, comparing observed and critical values, and Type I and Type II errors.
Sources & how we know this
- Eduqas GCE A Level in Psychology (A290) specification — Eduqas (2015)