Week 8: Experimental Designs (II)

• Repeated Measures Design

  The same participants experience all levels of the independent variable.
• What are the strengths and weaknesses of a repeated measures design?

  • Strengths: Fewer participants are needed and it is extremely sensitive to detecting differences between levels of the independent variable.
  • Weaknesses: The order of presenting the treatments may affect the dependent variable. This includes: practise, fatigue, and contrast effects.
• Practise Effects


  Progressive improvement in a performance as a participant gains experience through the series of treatment conditions.
• Fatigue Effects

  Progressive decline in performance as a participant works through a series of treatment conditions.
• Contrast Effects


  An example of a carryover effect in which the perception of a treatment condition is influenced by its contrast with the previous treatment.
• What are the solutions to order effects?

  Complete or partial counterbalancing; spacing time intervals.
• Complete Counterbalancing

  All possible orders of presentation are included in the experiment.

  
• How do we find out how many orders there are in complete counterbalancing?

  If there are X amount of conditions, it is continuously multiplied with descending ordinal conditions until it reaches the last condition.

  
• Partial Counterbalancing

  Each condition appears at each ordinal position and each condition precedes and follows each condition one time.

  
• Spacing Time Intervals

  The time between the first and second treatment condition. Used to counteract fatigue and contrast effects but more time between conditions may result in greater dropout because the study takes longer.
• Matched Pairs Design


  Researchers complete two steps to obtain equivalent experimental groups:

  1. They match participant on a key variable; they create pairs of participants.
  2. They randomly assign one of each pair to one level of the independent variable.

  This design is used less frequently as only a few participants are available and it is costly to run large numbers of people in an experiment.

  
• What are the differences between independent, repeated, and matched design?

  A) only

  B) one level

  C) independent

  D) Randomly assign

  E) all levels

  F) independent

  G) match two people

  H) characteristic

  I) randomly assign

  J) each pair

  K) only

  L) one level

  M) independent