9.1: Within-Subjects Experiments and Internal Validity

• What is the defining characteristic of a within-subjects design?

  It uses a single group of participants and tests; observes each individual in all of the different treatments being compared – the study repeats measurements of the same individuals under different conditions.
• What are the two threats to internal validity in the context of within-subjects experiments?

  1. Confounding from Environmental Variables – Characteristics of the environment that may change from one treatment condition to another; thus, there is an alternative explanation for the results.
  2. Confounding from Time-Related Variables – During the time between the first and final measurement, the participants may be influenced by a wide variety of factors other than the treatments being investigated.
• What are the five confounding time-related variables?

  1. History
  2. Maturation
  3. Instrumentation
  4. Statistical Regression
  5. Order Effects
• History

  Refers to environmental events other than the treatment that changes over time and may affect the scores in one treatment differently than in another treatment.
• Maturation

  Any systematic changes in participants' physiology or psychology that occur during a research study and affect the participants' score.
• Instrumentation

  Refers to changes in a measuring instrument that occur over time.
• Statistical Regression

  Refers to the tendency for extreme scores on any measurement to move toward the mean when the measurement procedure is repeated.
• Order Effects

  Occurs when the experience of being tested in one treatment condition has an influence on the participants' score in a later treatment condition.
• Carry-over Effects

  Sub-effect of order effects; changes in the score observed in one treatment condition that are caused by the lingering aftereffects of a specific earlier treatment condition.
• Contrast Effect

  Sub-effect of order effects; an example of a carryover effect in which the perception of a treatment condition is influenced by its contrast with the previous treatment.
• Progressive Error

  Sub-effect order; changes in the scores observed in one treatment condition that are related to general experience in a research study over time, but not to a specific treatment or treatments.
• Fatigue Effects

  Sub-effect of progressive error; progressive decline in performance as a participant works through a series of treatment conditions.
• Practise Effects

  Sub-effect of progressive error; progressive improvement in a performance as participant gains experience through the series of treatment conditions.