Experimental Designs

• Experimental designs only apply to experiments.
• Experimental Design = The different ways in which the testing of participants can be organised in relation to the experimental conditions.
• Independent Measures/Groups:

  1. Recruit a group of participants and divide them into two.
  2. One group does the experimental task with the IV set for condition 1.
  3. The other group does the experimental task with the IV set for condition 2.
  4. Measure the DV for each group and compare results for the 2 groups.
• Independent Measures/Groups = 2 separate groups of participants experience the 2 different conditions of the experiment.
• Repeated Measures:

  1. Recruit a group of participants
  2. The group does the experimental task with the IV set for condition 1.
  3. The group then repeats the experimental task with the IV set for condition 2.
  4. Compare the results for the 2 conditions.
• Repeated Measures = All participants experience both conditions of the experiment. Each participant would first experience condition A. At a later date, they would all experience condition B.
• Matched Pairs = Participants are paired together on a variable or variables that are relevant to the experiment.
• Matched Pairs:

  1. Recruit a group of participants to find out what sorts of people you have in the group.
  2. Recruit another group that matches them one for one.
  3. Treat the experiment as independent measures.
  4. Compare the results for the matched pairs.
• How can we control participant variables?


  Random allocation - Participants are randomly allocated to the different experimental conditions.

  • This is especially useful for independent group designs.
• Single Blind Technique:


  Participants do not know the aim or hypothesis of the experiment.
• Double blind Technique: 


  Neither the participants nor the experimenter know the aim or hypothesis of the experiment.
• How can we control order effects?


  Counterbalancing - Half of the participants do condition A then condition B and the other half do condition B and the condition A.
• Independent Group Design - Advantages:

  • Avoids order effects as people participate in one condition only.
• Repeated Group Design - Advantages:

  • Fewer people are needed as they take part in all conditions.
• Matched Pairs Design - Advantages:

  • Reduces participant variables because researcher has tried to pair up participants so that each condition has people with similar abilities and characteristics.
  • Avoids order effects
• How to control participant variables:

  • Random allocation
  • Single-blind technique
  • Double-blind technique
• Order effects = e.g. getting bored/tired so getting worse at the task (fatigue effects) or people getting better at the task because of practice (practice effects).
• Independent Group Design - Disadvantages:

  • More people needed than with the repeated measures design - more time consuming.
  • Differences between participants in the groups may affect results (participant variables).
• Repeated Group Design - Disadvantages:

  • There may be order effects.
• Matched Pairs Design - Disadvantages:

  • Very time consuming to find closely matched pairs.
  • Impossible to match people exactly, unless they are identical twins.