What are the pros and cons of a within-subjects design?

Within-subjects designs have many potential threats to internal validity, but they are also very statistically powerful.

Advantages:

  • Only requires small samples,
  • Statistically powerful,
  • Removes the effects of individual differences on the outcomes.

Disadvantages:

  • Internal validity threats reduce the likelihood of establishing a direct relationship between variables,
  • Time-related effects, such as growth, can influence the outcomes,
  • Carryover effects mean that the specific order of different treatments affect the outcomes.