experimental design

• experimental design is how levels of the IV are delivered.
• independent groups uses a different group per condition. involves two or more seperate sets who represent just one condition of the IV.
• repeated measures


  uses same participants in all conditions
• matched pairs

  involves two sets of participants similar to independent measures but they are matched in pairs based on a pre test of a key variable e.g. score on a test. pairs are then compared rather than whole groups.
• advantage of independent groups

  no order effects
• disadvantage of independent groups

  more participant variables

  more costly
• using random allocation helps overcome participant variables in independent groups.
• advantages of repeated measures

  high validity because participant variables are controlled
• disadvantages of repeated measures

  1. high order effects (getting better/bored at something the 2nd, 3rd time which results in better/worse results)
  2. demand characteristics due to participants guessing the study
• a way to get rid of order effects in repeated measures is counter balancing, where half of the group does condition A then B, and the other does B then A.
• advantages of matched pairs

  1. removes order effects/DCs as pairs only do one condition
  2. reduces participant variables, higher internal validity
• disadvantage of matched pairs

  1. difficult to match participants on all important key characteristics
• one way to get around difficulty of matching pairs on key characteristics is pilot studies.
• how to overcome order effects:

  1. counterbalancing
  2. randomisation
  3. random allocation
• randomisation 


  material for each experiment is placed in a random order for the participant. e.g. the same words are presented but in a different order for each participant
• random allocation


  pulling names out of a hat. a non-biased method to allocate participants to experimental and control conditions in an experiment in independent groups design.