Experimental designs: repeated measures, independent groups, matched pairs.

The idea

With repeated measures the same people take part in every condition, so participant variables are held constant and half as many people are needed. The price is order effects such as practice, fatigue and boredom, which counterbalancing addresses, and a higher risk of demand characteristics. In an independent groups design a different set of people does each condition: no order effects, but individual differences now sit between the groups and could produce the result on their own, so allocation is randomised and twice as many participants are recruited. A matched pairs design tests different people who have been paired on variables relevant to the DV, such as age or IQ, splitting each pair across conditions, which takes time and is never perfect outside identical twins.

What earns the marks

• Describes each design in terms of who does which condition.
• Gives the strength of repeated measures as control of participant variables and fewer participants needed.
• Gives the limitation of repeated measures as order effects, with counterbalancing named as the control, and demand characteristics.
• Gives the independent groups trade-off: no order effects, but participant variables between groups, controlled by random allocation.
• Describes matched pairs as pairing on a relevant variable, combining the benefits, but time-consuming and imperfect.

Common mistakes and how to put them right

• Saying matched pairs uses the same participants twice.

  Put it right: It uses different people who have been paired.

• Claiming counterbalancing removes order effects rather than balancing them across conditions.

• Claiming counterbalancing controls demand characteristics.

  Put it right: It only addresses order effects.

• Listing textbook strengths that do not fit the scenario, for example recommending repeated measures when the task cannot be repeated.

• Confusing random allocation, which assigns people to conditions, with random sampling, which selects them in the first place.

Exam tips

• Justification questions want the reason tied to the stem: if the task cannot be done twice, say so and choose independent groups.
• Name the specific control for the weakness you raise, because the mark is usually for the remedy.
• In a design question, state the design, then the conditions, then how participants are allocated.