Like between-subjects designs, correlated-groups designs may use more than two levels of an independent variable.You should remember from Chapter 9 that there are two types of correlated-groups designs: a within- subjects design and a matched-subjects designs.The same statistical analyses are used for both designs. We will use a within-subject designs to illustrate the statistical analysis appropriate for a correlated-groups design with more than two levels of an independent variable.
Imagine now that we want to conduct the same study as before, on the effects of rehearsal type on memory, but using a within-subjects rather than a between-subjects design.Why might we want to do this? As such, within-subjects designs-in fact, all correlated-groups designs are more powerful than between-subjects designs.
Therefore, one reason for this choice is to increase statistical power. In addition, the within-subjects design uses fewer participants and provides almost perfect control across conditions.Because the same people participate in each condition, we know that the individuals in each condition are equivalent and that the only difference between conditions is the type of rehearsal used.In this study, the same three conditions will be used-rote rehearsal,rehearsal with imagery, and rehearsal with a story.The only difference is that the same eight subjects serve in every condition.Obviously, we cannot use the same list of words across conditions because there could be a large practice effect.We therefore have to use three lists of words that are equivalent in difficulty and that are counterbalanced across conditions. In other words, not all participants in each condition will receive the same list of words.