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I am trying to use a repeated measures ANCOVA to look at differences over time between groups in SPSS. I was following this helpful thread (What is the correct way to add and analyse covariates in a 2x3 Mixed ANOVA in SPSS), but the example only uses two timepoints.

Does anyone know how it might be extended to involve a third timepoint? I am less confident with syntax for this analysis, and when I try to use the GUI to run a GLM RM analysis, I can't enter the baseline as my first timepoint (within-subjects box) as well as enter it as a covariate.

I am getting familiar with R too, so any suggestions for R code would be great too.

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  • $\begingroup$ Why are you using RM ANCOVA? It makes some very strong assumptions. I suggest looking into multilevel models. $\endgroup$
    – Peter Flom
    Commented Jun 21, 2023 at 12:39
  • $\begingroup$ Thanks, I would look at longitudinal mixed models but I'm lacking a guide for how to conduct either in SPSS or R, including how to include the baseline as a covariate. $\endgroup$
    – Biscuity
    Commented Jun 21, 2023 at 13:30
  • $\begingroup$ There are lots of books, at least for doing it a) In general and b) In R. The one by Gelman and Hill is quite good. In an MLM, you consider all the time points. You don't add the baseline as covariate. There's also lots of threads here on CV about MLMs. And an "R Bootcamp" from Penn State. I haven't looked at it, but Penn State has a good rep. $\endgroup$
    – Peter Flom
    Commented Jun 21, 2023 at 19:08

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