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This means that the population variances of all possible difference scores (com_1 - com_2, com_1 - com_3 and so on) are equal. However, this assumption is not needed if the sample size >= 25.

The test variables follow a multivariate normal distribution in the population.This is often -not always- satisfied by each case in SPSS representing a different person or other statistical unit. Independent observations (or, more precisely, independent and identically distributed variables).For a (single factor) repeated measures ANOVA these are Running a statistical test doesn't always make sense results reflect reality only insofar as relevant assumptions are met. Reversely, our sample means being slightly different is a normal sample outcome if population means are all similar. We'll reject this if our sample means are very different. The 4 population mean scores are all equal.

We'll then try and generalize this sample outcome to our population by testing the null hypothesis that The research question is: which commercial has the highest mean rating? We'll first just inspect the mean ratings in our sample. Although such ratings are strictly ordinal variables, we'll treat them as metric variables under the assumption of equal intervals. She shows the four concepts to 40 participants and asks them to rate each one of them on a 10-point scale, resulting in commercial_ratings.sav. Repeated Measures ANOVA ExampleĪ marketeer wants to launch a new commercial and has four concept versions. Whatever distinguishes these variables (sometimes just the time of measurement) is the within-subjects factor. The simplest repeated measures ANOVA involves 3 outcome variables, all measured on 1 group of cases (often people). Large sample differences, however, are unlikely these suggest that the population means weren't equal after all. If this is true and we inspect a sample from our population, the sample means may differ a little bit. SPSS repeated measures ANOVA tests if the means of 3 or more metric variables are all equal in some population.