EXPERIMENTAL DESIGN

• Definition – the way participants are used in experiments 
• Independent group design (IGD) – participants are allocated to different groups where each group represents a different experimental condition e.g. experimental and control 
• Strengths of IGD

  • Order effects are not a problem 
  • Reduced demand characteristics – participants are less likely to guess aim 
• Weaknesses of IGD

  • Participant variables may act as a confounding variable  
  • More participants are needed – more expensive? 
• IGD:

  Main problem: 

  -Lots of individual differences (participant variables) - CAGE, personality, intellect 

  Solution: 

  -Randomisation – no researcher bias (lottery, computer generated) 
• Repeated measure design (RMD) – all participants take part in all conditions of the experiment 
• Strengths of RMD

  • Participant variables are controlled, and fewer participants are needed – higher validity 
  • Performance may improve from task to task 
• Weaknesses of RMD

  • Order effects are a problem due to the participants doing all the tasks – can solve it by counterbalancing  
  • More likely to work out aim when taking part in all aspects of the experiment – demand characteristics 
• RMD:

  Main problem: 

  -Order effects (practice, boredom, fatigue) 

  Solution: 

  -Counterbalancing – attempts to balance out order effects (person 1 does condition A then B and person 2 does B then A) 
• Matched pair design (MPD) – pairs of participants are first matched on some variable(s) that may affect the DV (Dependent Variable) and each person in the pair is assigned to one condition. 
• Strengths of MPD

  • Order effects and demand characteristics are less of a problem as each participant is only taking part in one condition 
• Weaknesses of MPD

  • Participants can never be matched exactly so participant variables may still affect the DV 
  • Time consuming and expensive