Experimental Design

• What must the researcher consider when making an appropriate design?

  • the precise nature of the experimental task
  • how to control the relevant variables
  • the availability of participants
• What is the Independent groups design?

  Different participants used in each condition of the experiment.

  Design typically involves:

  • A control and one or more experimental conditions
  • Two or more experimental conditions
  • Participants are randomly allocated to conditions to try to ensure that participant variables do not differ systematically between conditions
• What is the Design Repeated Measures?

  The same participants take part in each condition of the independent variable. This means that each condition of the experiment includes the same group of participant.
• What is the design Matched Pairs?

  Each participant in one group/condition is carefully matched on all the variables considered to be relevant to the investigation with a participant in another group/condition.
• What are the Strengths of Independent groups?

  • Avoids order effects (such as practice or fatigue) as people participate in one condition only. If a person is involved in several conditions they may become bored, tired and fed up by the time they come to the second condition, or becoming wise to the requirements of the experiment.
  • Wide range of potential uses.
• What are the Limitations of Independent groups?

  • Requires twice as many participants as a repeated measures design (more time consuming)
  • Differences between participants in the groups may affect results, for example: variations in age, gender or social background. These differences are known as participant variables (i.e. a type of extraneous variable)
• What are the Strengths of Repeated Measures?

  • No individual differences between participants in different conditions
  • Fewer people are needed as they take part in all conditions (i.e. saves time)
• What are the Limitations of Repeated Measures?

  • Smaller range of potential uses compared to the independent design.
  • There may be order effects (the order of the conditions having an effect on the participants behaviour). Performance in the second condition may be better because the participants know what to do (i.e. practice effect). Or their performance might be worse in the second condition because they are tired (i.e. fatigue effect). - This can be addressed through counterbalancing or randomisation.