experimental design

• experimental design refers to how participants are allocated to the different conditions
• there are three types of experimental design: independent groups, repeated measures and matched pairs
• in independent groups design the participants take part in one condition only, each condition has a different group of participants
• advantages of independent groups:

  • less chance of order effects
  • reduces risk of demand characteristics as participants are only in one condition so are less likely to guess the aim of the study
  • can be used when repeated measures design in inappropriate (e.g. gender) as each condition needs different participants
• disadvantages of independent groups:

  • participant variables may affect the DV as there are separate groups of participants in each condition so it can lower internal validity
  • more participants are needed as 2 different groups are required
• in repeated measures design all participants take part in all conditions of the experiment
• advantages of repeated measures:

  • participant variables between participants are removed
  • requires fewer participants as the same participants take part in all conditions, it may increase population validity
• disadvantages of repeated measures:

  • increased chance of demand characteristics as participants take part in all conditions so may notice clues and change their behaviour lowering internal validity
  • order effects may occur as participants take part in both conditions so might get bored or tired
  • Inappropriate to use for quasi-experiments where participants are unable to take part in all conditions
• in matched pairs design, participants are matched on important characteristics (e.g. age or IQ), one member of the pair is then put in each condition
• advantages of matched pairs:

  • controls the participant variables
  • low chance of order effects as the participants will only do one condition
• disadvantages of matched pairs:

  • very time consuming to match up and find similar participants
  • difficult to completely match participants on all important characteristics so they may still be some participant variables