Table 13-4 (p. 403) shows that the within-subjects design is also threatened by something that does not affect matched pairs designs: order effects. Specifically, practice, fatigue, carry-over, and sensitization effects can affect the results of a within-subjects experiment.
To give students a clearer picture of the problem with order effects--and to suggest ways of dealing with order effects, go over Table 13-5 (p. 406). Show how order effects could affect the results of a a within-subjects design (such as the one discussed in the text and referred to in Table 13-4). Finally, talk about randomization and counterbalancing combined with randomization as ways of dealing with order effects
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