Experiments: Experimental Design (OCR GCSE Psychology): Revision Note

Experimental design

• Before carrying out an experiment, psychologists must decide how participants will be used across the different conditions of the IV

• This is known as the experimental design — it shows how the participant sample is organised to test each condition

• If an experiment has two conditions, participants might:

  • take part in both conditions (repeated measures design), or

  • take part in only one condition (independent groups design),

• The main types of experimental design are:

  • independent groups design

  • repeated measures design

Independent groups design

• In an independent groups design, participants take part in only one condition of the IV

  • This means different participants are used in each condition, and their performance or behaviour is compared

• Participants are randomly allocated to each condition to avoid researcher bias

• This design produces unrelated data because the scores in each condition come from different individuals

Example of an independent groups design

• In a study on memory and noise:

  • Participant A learns a poem with music playing (Condition 1)

  • Participant B learns the same poem in silence (Condition 2)

  • Each participant is then asked to recall as many lines as possible from the poem within five minutes

• The researcher then compares the average recall scores between the two groups to see whether the IV (music or silence) affects the DV (number of lines correctly recalled)

Evaluation of an independent groups design

Strengths

• There are no order effects (e.g. boredom, fatigue, or practice effects) because each participant takes part in only one condition

• Participants are less likely to show demand characteristics — they are less likely to guess the aim of the study

• The same materials can be used across both conditions without participants becoming familiar with them

Weaknesses

• Individual differences may affect results — one group might naturally have better memory, attention, or ability than the other

  • Differences between conditions might be due to participant variables rather than the IV

• More participants are required, as each person only takes part in one condition

Repeated measures design

• In a repeated measures design, the same participants take part in all conditions of the IV

  • This means each participant’s performance in one condition can be directly compared to their performance in the other condition

• The data produced are known as related data, because the two sets of results come from the same individuals

• Participants act as their own control group, helping to eliminate the influence of individual differences such as memory ability, motivation, or intelligence

Example of a repeated measures design

• In a study on memory and noise:

  • Participant A learns a poem with music playing (Condition 1)

  • Participant A then learns a different poem in silence (Condition 2)

  • After each condition, they are asked to recall as many lines as possible from the poem within five minutes

• The researcher compares how many lines each participant recalls across both conditions to see whether the IV (music or silence) affects the DV (number of lines correctly recalled)

Dealing with order effects: counterbalancing

• Because the same participants complete all conditions, their performance may be affected by order effects, such as:

  • practice effects: performing better the second time due to familiarity with the task.

  • fatigue or boredom effects: performing worse the second time due to tiredness or loss of motivation

• To control for this, researchers use counterbalancing:

  • Participants are split into two groups

    • Group 1 completes Condition A first, then Condition B

    • Group 2 completes Condition B first, then Condition A

• Counterbalancing ensures that order effects are evenly distributed across conditions, reducing their impact on the results

  • They are not eliminated completely

Evaluation of a repeated measures design

Strengths

• Individual differences are controlled because the same participants take part in both conditions

• Fewer participants are needed, making the study more efficient and cost-effective

Weaknesses

• Order effects (practice, fatigue, boredom) may influence participants’ performance in later conditions

• Demand characteristics are more likely, as participants may guess the aim of the study after completing one condition