Experimental design

• Independent groups design is when different participants take part in each condition of the experiment. They are randomly allocated to these conditions to avoid researcher bias.
• Independent groups design produces unrelated data (data points in one condition cannot be paired with any of the data points in the other condition).
• Limitations of the independent groups design:

  • Participant variables - If more participants with a particular characteristic are randomly assigned to one of the groups (eg age). This can influence the measurement of the DV (an extraneous variable).
• Strengths of the independent groups design:

  • No order effects so there is increased validity and it can save time, money and effort because both groups can simultaneously go through testing.
• Repeated measures design is when the same participants take part in each condition of the study.
• Repeated measures design produces related data as each participants score (data point) in one condition can be paired with another data point in their own score in the other condition.
• Strengths of the repeated measures design:

  • No participant variables because the same participants take part in each condition.
• Limitations of the repeated measures design:

  • Order effects - Taking part in the first condition influences performance in the second condition. This may be worsened performance due to fatigue.
  • Demand characteristics - Participants are more likely to figure out the aim and alter their behaviour.
• Counter balancing attempts to control the order effects by using the ABBA format. Half of the participants take part in condition A and B while the others start with B and then A. However, this does not eliminate the order effects.
• Matched pairs design is when different participants complete in each of the conditions. They are matched by characteristics, age, gender etc. Each of the two in the pair are randomly allocated to their conditions.
• Matched pairs design produces related data. Each participant score a data point which can be compared with the other condition.
• Strengths of the matched pairs design:

  • Reduced participant variables - Participants matched on relevant characteristics.
  • No order effects as participants take part in each condition once.
• Limitations of matched pairs design:

  • Longer to set up.
  • Twice as many participants required compared to the repeated measures design.
  • Some participant variables - Not identical participants.