Experimental design

• What is the experimental design?

  The different ways in which ppts can be organised in relation to the experimental conditions
• Types of experimental designs

  • Independent groups design
  • Repeated measures
  • Matched-pairs design
• What is the independent groups design?

  Ppts are allocated to diff groups where each group represents a diff experimental condition
• What is repeated measures?

  All ppts take part in all conditions of the experiment (all experimental conditions and the control condition)
• What is the matched pairs design?

  Pairs of ppts are first matched on some variable(s) that may affect the dependent variable, then one of each pair is allocated to a diff condition
• Advantages of independent groups

  • Quic,k and efficient
  • Good variety of people
  • Random allocation allows even distribution of ppts characteristics across the conditions
• Disadvantages of independent groups design

  • Ppt variables may act as a confounding variable reducing validity of findings (problem is dealt w/ random allocation)
• Advantages of repeated measures

  • To balance out order effects the researcher uses counterbalancing
  • No ppt variables
  • Fewer ppts + cheaper
  • On a skill-based task, after doing the first task, the ppts peformance may improve by practice and they would perform better on the second task
• Disadvantages of repeated-measures

  • Each ppt has to do at least two tasks
  • There are order effects (order in which the tasks are done is significant) - problem is dealt with counterbalancing
  • Repeating two tasks could create fatigue that may cause deterioration in performance on the second task
  • By experiencing all conditions of the study, ppts are more like to work out the aim so demand characteristics will arise
• What is counterbalancing?

  Half of the ppts do condition A first then B, the other half do B, then A and then the two groups switch so that every ppt does the two conditions again but in the opposite order
• Advantages of matched pairs

  • No order effect because ppt only goes through one condition
  • Less likely to be demand characteristics
• Disadvantages of matched pairs

  • Ppts can never be matched exactly and differences may affect the DV
  • Matching may be time-consuming and time consuming if a pre-test is required = less economical than other designs