Experimental designs

• Independent groups

  • When 2 separate groups of participants experience 2 different conditions of the experiment.
  • Participants should be randomly allocated.
• Advantages of independent groups

  • Order effects aren’t a problem as participants are only tested once so can’t practice or become bored, which controls an important confounding variable.
  • Participants are less likely to guess the aim of the study as they are only tested once so behaviour may be more ’natural’.
• Disadvantages of independent groups

  • Participants in the 2 groups are not the same in terms of participant variables. This acts as extraneous/confounding variables which may reduce the validity of the study.
  • They are less economical as twice as many participants are needed to produce the same data as in a repeated measures design. This increases the time and money spent on recruiting participants.
• Repeated measures

  When all participants experience both conditions of the experiment.
• Advantages of repeated measures

  • Participant variables are controlled as the person in both conditions have the same characteristics, which controls an important confounding variable.
  • Fewer participants are needed as all participants do both conditions, so less time is spent recruiting participants.
• Disadvantages of repeated measures

  • Order effects are a problem as participants have to do at least 2 similar tasks which could create boredom and participants performance may improve through practising. This reduces the validity of the results.
  • Participants may also be able to guess the aims of the study as they experience all conditions of the experiment. This can reduce the validity of the results as they may respond to demand characteristics.
• Matched pairs

  Participants are paired together based on participant variables and then allocated to a different condition than the other pairs.
• Advantages of matched pairs

  • Participants are matched based on a variable that is relevant to the experiment. This controls participant variables and enhances the validity of the results.
  • No order effects as participants are only tested once so wont get bored, which enhances the validity of the results.
• Disadvantages of matched pairs

  • Matching participants isn’t perfect as it’s time-consuming and its‘s not possible to control all the relevant variables.
  • Twice as many participants are needed compared to the repeated measures design, so more time and money is spent on recruiting participants.