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Assumptions in Repeated Measures ANOVAExcept for independence of samples, the assumptions for simple ANOVA, (betweensubjectsdesigns), discussed in chapter 9 also hold true for repeated measures ANOVA,(within-subjects designs). But with repeated measures designs, we must also considerthe relationships among the repeated measures. Repeated measures ANOVAmust meet the assumption of sphericity, sometimes referred to as compound symmetry.Sphericity requires that the repeated measures demonstrate homogeneity ofvariance (see chapter 8, assumptions for the t test) and homogeneity of covariance.Homogeneity of covariance means that the relationships, or correlations, on thedependent variable among all of the three or more repeated measures are equal.When only two repeated measures are ernployed (such as pre-post measures for a ttest), this assumption is not applicable, because there is only one correlation coeffcientthat can be calculated, the correlation between pre- and posttest scores. Methodsof dealing with violations of sphericity will be presented later in this chapter
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