Experimental designs and controls

The three designs

• Repeated measures: the same participants do every condition. No participant variables between conditions and fewer participants needed; but order effects (practice, fatigue, boredom) and participants may guess the aim. Control: counterbalancing.
• Independent groups: different participants in each condition. No order effects; but participant variables (differences between people) may affect results and more participants are needed. Control: random allocation.
• Matched pairs: different participants, matched in pairs on relevant variables (age, IQ, memory span), one of each pair in each condition. No order effects and fewer participant variables; but matching is time-consuming and never perfect.

Controls

• Counterbalancing: half the participants do condition A then B, the other half B then A (ABBA), so order effects are balanced across conditions.
• Randomisation: using chance to allocate participants to conditions or to order the stimuli, so bias is spread evenly.
• Control group: a group that does not receive the IV, giving a baseline for comparison.
• Standardised procedures and instructions: everyone experiences the same thing.

Situational and participant variables

• Situational variables: features of the environment that could affect the DV (noise, temperature, lighting, time of day). Control by keeping them the same for all participants.
• Participant variables: characteristics of the participants (age, intelligence, motivation, memory ability). Control with repeated measures, matched pairs or random allocation.

Worked example

A researcher tests memory with music and without music using the same participants. Explain one problem and how to control it.

1. Problem: order effects. In the second condition participants may do better from practice or worse from fatigue.
2. Control: counterbalance. Half do music then silence, half silence then music.
3. Use different but equally difficult word lists in each condition.

Answer: Order effects: counterbalance the conditions (ABBA) and use equivalent word lists.

Check yourself

1. What is the main problem with a repeated measures design?
   • Participant variables between conditions
   • Order effects such as practice and fatigue
   • It needs twice as many participants
   • Matching takes a long time
   Show answer

   Order effects such as practice and fatigue. The same people do both conditions, so order can affect results.

2. Which control is used to reduce participant variables in an independent groups design?
   • Counterbalancing
   • Using the same participants twice
   • Changing the IV
   • Random allocation to conditions
   Show answer

   Random allocation to conditions. Random allocation spreads individual differences evenly.

3. Half the participants do condition A then B; the other half do B then A. This is
   • random sampling
   • counterbalancing
   • matched pairs
   • a control group
   Show answer

   counterbalancing. ABBA balances order effects.

4. Which is a situational variable?
   • A participant's age
   • A participant's IQ
   • The noise level in the testing room
   • A participant's motivation
   Show answer

   The noise level in the testing room. Situational variables are features of the environment.

5. In a matched pairs design, participants are
   • paired on relevant characteristics, and one of each pair goes into each condition
   • tested in every condition
   • allocated with no regard to their characteristics
   • always identical twins
   Show answer

   paired on relevant characteristics, and one of each pair goes into each condition. Matching reduces participant variables without order effects.

6. Explain why a researcher might choose a repeated measures design for a memory experiment.
   Show answer

   Each participant acts as their own control, so individual differences in memory ability (participant variables) cannot explain differences between conditions, and fewer participants are needed.. Repeated measures removes participant variables between conditions.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification (issue January 2026)
• Pearson International A Level Psychology component guide: WPS01
• IAL Psychology revision site: Experimental designs
• IAL Psychology revision site: Controls

Last updated 8 November 2026. Spotted a mistake? Tell us.