Methodology: The same group of participants takes part in every condition of the independent variable. The researcher compares each individual's performance in one condition against their own performance in the others.
Strengths: This design effectively eliminates participant variables because the same people are used in all groups, acting as their own control. It also requires fewer participants to generate the same amount of data.
Weaknesses: It is highly susceptible to order effects, where the sequence of tasks influences the outcome. For example, a participant might perform better in the second condition due to practice, or worse due to fatigue.
Counterbalancing: To combat order effects, researchers use counterbalancing (e.g., the ABBA technique), where half the participants do Condition A then B, while the other half do Condition B then A.
Methodology: Participants are pre-screened and paired based on specific characteristics relevant to the study (e.g., age, gender, or skill level). One member of each pair is then randomly assigned to Condition A and the other to Condition B.
Purpose: This design attempts to combine the benefits of both independent groups and repeated measures by reducing participant variables without introducing order effects.
Limitations: It is the most time-consuming and difficult design to implement, as finding closely matched pairs requires extensive pre-testing and a large initial pool of participants.
| Feature | Independent Groups | Repeated Measures | Matched Pairs |
|---|---|---|---|
| Participants | Different people per condition | Same people for all conditions | Different but similar people |
| Order Effects | None | High risk (Practice/Fatigue) | None |
| Participant Variables | High risk of bias | Controlled (Eliminated) | Reduced/Controlled |
| Sample Size | Large required | Smallest required | Large required |
Identify the Design: When reading a scenario, ask: 'Does each person do the task once or twice?' If once, it is Independent Groups or Matched Pairs. If twice, it is Repeated Measures.
Justify the Choice: If asked to evaluate a design, always link your answer to the specific variables in the study. For example, if the task is a memory test, mention that 'practice effects' are a specific risk for Repeated Measures.
Check for Counterbalancing: If a study uses Repeated Measures, always check if the researcher mentioned counterbalancing. If they didn't, it is a major methodological weakness you can critique.
Participant Variables: In Independent Groups, always suggest random allocation as the primary way to deal with individual differences between groups.