PSYC 3550 Chapter Notes - Chapter 9: Confounding, Repeated Measures Design, Regression Analysis
The defining characteristic of a between-subjects experiment is that it requires separate but equivalent groups of participants for the different treatment conditions compared. The defining characteristic of a within-subjects design is that it uses a single group of participants, and tests or observes each individual in all of the different treatments being compared. The sample is not separated into several groups but rather exists as a single group that participated in every treatment condition. In a within-subjects design the same group of individuals participated in every level of the independent variable so that each participant experiences all of the different levels of the independent variable. In one sense, a within-subjects study is the ultimate in equivalent groups because the group in one treatment condition is absolutely identical to the group in every other condition. Figure 9. 1 [the structure of a within-subjects design] the same sample of individuals participates in all of the treatment conditions.