Component 2: Experimental design (knowledge, understanding and evaluation of) — Eduqas A-Level Psychology

Experimental design in Eduqas A-Level Psychology covers the key decisions researchers make when planning studies: choosing experimental designs (independent measures, repeated measures, matched pairs), controlling extraneous and confounding variables, allocating participants to conditions, and selecting appropriate experimental designs to test hypotheses. Understanding these choices is essential for evaluating validity, reliability and the strength of causal conclusions drawn from psychological research.

Experimental design is the set of decisions a researcher makes about how participants are allocated to conditions and how variables are controlled. In Eduqas A-Level Psychology, you must know the three main designs (independent measures, repeated measures, matched pairs), their strengths and limitations, and how they affect validity and reliability. You also need to understand how to control extraneous and confounding variables through standardisation, random allocation, counterbalancing and matching.

This topic matters because the quality of an experiment depends on its design. A poorly designed study cannot support causal conclusions, no matter how interesting the hypothesis. Experimental design connects to research methods, data analysis and the evaluation of classic studies across all psychology topics, so examiners frequently test it in both short-answer and extended-response questions.

• →Independent measures design: different participants in each condition; avoids order effects but risks participant variables.
• →Repeated measures design: same participants in all conditions; controls participant variables but risks order effects and demand characteristics.
• →Matched pairs design: participants matched on key variables and allocated to different conditions; reduces participant variables and order effects but matching is imperfect and time-consuming.
• →Extraneous variables: unwanted variables that may affect the dependent variable if not controlled; confounding variables systematically vary with the independent variable and undermine causal conclusions.
• →Control techniques: random allocation, counterbalancing, standardisation, single-blind and double-blind procedures, and matched pairs.

• 💡Always name the experimental design explicitly and then link each strength or limitation to validity, reliability or causal conclusions. Generic answers without a link rarely gain full marks.
• 💡When asked to improve a study, suggest a specific design change (for example, 'use a matched pairs design and match on age and baseline reaction time') rather than vague advice like 'control the variables better'.
• 💡Use precise terminology: participant variables, order effects, counterbalancing, standardisation, demand characteristics, internal validity. Examiners reward correct technical language.

• Students often think repeated measures is always better because it uses the same participants. In fact, repeated measures introduces order effects and demand characteristics, so counterbalancing is essential and the design is not automatically superior.
• Students sometimes say that random allocation removes all extraneous variables. Random allocation only balances participant variables across conditions on average; it does not control situational variables, so standardisation is still needed.
• Students confuse extraneous and confounding variables. Extraneous variables add noise; confounding variables change systematically with the IV and make it impossible to isolate the cause of the effect.

1. 1Day 1-2: Create a comparison table of independent measures, repeated measures and matched pairs designs, listing one strength and one limitation for each with a concrete example.
2. 2Day 3-4: Learn definitions and examples of extraneous and confounding variables, and make a list of control techniques with a specific example of each.
3. 3Day 5-6: Practise applying designs to novel scenarios. For each scenario, identify the design, suggest one improvement and explain the effect on validity.
4. 4Day 7-8: Complete past-paper questions on experimental design, focusing on 4- and 6-mark questions. Mark your answers against the mark scheme and rewrite any that lost marks.
5. 5Day 9-10: Use active recall and spaced practice to memorise key terms and evaluate at least two classic studies in terms of their experimental design.

• 📋Short-answer definition questions (2-3 marks): Define independent measures, repeated measures or matched pairs, and give one strength or limitation. Advice: be precise and use an example.
• 📋Application questions (4-6 marks): Read a scenario and identify the design, explain one strength and one limitation, or suggest an improvement. Advice: always link your point back to the scenario and to validity.
• 📋Evaluation questions (6-8 marks): Discuss the strengths and limitations of a named design or compare two designs. Advice: use a balanced structure with clear topic sentences and psychological terminology.
• 📋Design-a-study questions (6-10 marks): Outline how you would design an experiment to test a hypothesis, including design, controls and participant allocation. Advice: cover design, variables, controls, sampling and ethics briefly but accurately.