The one thing that doesn’t is to combine the two approaches. The person she’d asked for advice was in a medical field, and had been trained on the ANCOVA approach.Įither approach works well in specific situation. (For the record, linear mixed models also work, and had some advantages, but in this design, the results are identical).
It’s very common in medical studies because the focus there is about the size of the effect of the treatment.Īs it turned out, the right analysis to accommodate Nancy’s design and answer her research question was the Repeated Measures ANOVA. So when the research question is about the difference in means at post-test, this is a great option. The other is to account for variation around the post-test means that comes from the variation in where the patients started at pretest. One is to make sure that any post-test differences truly result from the treatment, and aren’t some left-over effect of (usually random) pre-test differences between the groups. The adjustment for the pre-test score in ANCOVA has two benefits. It’s appropriate when the research question is not about gains, growth, or changes. In the ANCOVA approach, the whole focus is on whether one group has a higher mean after the treatment. The ANCOVA approach answers a different research question: whether the post-test means, adjusted for pre-test scores, differ between the two groups. This is directly measured by the time*group interaction term in the repeated measures ANOVA.
Nancy’s research question was whether the mean change in the outcome from pre to post differed in the two groups. Sometimes they’re right, but sometimes, as was true here, the two analyses answer different research questions. This kind of situation happens all the time, in which a colleague, a reviewer, or a statistical consultant insists that you need to do the analysis differently. The more she insisted repeated measures didn’t work in Nancy’s design, the more confused Nancy got. The advisor said repeated measures ANOVA is only appropriate if the outcome is measured multiple times after the intervention. This model assesses the differences in the post-test means after accounting for pre-test values. The pre-test measure is not an outcome, but a covariate. In ANCOVA, the dependent variable is the post-test measure. The advisor insisted that this was a classic pre-post design, and that the way to analyze pre-post designs is not with a repeated measures ANOVA, but with an ANCOVA. Nancy was sure that this was a classic repeated measures experiment with one between subjects factor (treatment group) and one within-subjects factor (time). Nancy had measured a response variable at two time points for two groups: an intervention group, who received a treatment, and a control group, who did not.īoth groups were measured before and after the intervention. Nancy was sure repeated measures was appropriate and the response led her to fear that she had grossly misunderstood a very basic tenet in her statistical training. The advisor told Nancy that actually, a repeated measures analysis was inappropriate for her data.
#How to test the homogeneity of slopes by spss version 25 how to#
Nancy had asked for advice about how to run a repeated measures analysis. Not too long ago, I received a call from a distressed client.
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