Repeated Measures ANCOVA

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    Dear Andy,

    I am a PhD student from Switzerland, and I have a stats question regarding Anova/Ancova, and I hope you cam help me (I couldn’t find an answer to this in your book):

    Each 2 participants per trial received the same 2 interventions one after another. I have a mixed factorial design (within variable: intervention, 2 levels; between variable: role of subject, 2 levels). I measured participants mood at baseline level, after the first intervention and after the second intervention. I want to compare participant’s mood after the two interventions (and also whether the role of the participant had an influence on mood), so my repeated measures variable has 2 levels. Also, i want to take into account the baseline level of mood, so i thought to include the baseline assessment as covariate in the repeated measures ANOVA (to make it a repeated measures ANCOVA).
    I don’t know how to interpret the output. Does the covariate influence both factors, between and within? And why is there an interaction term in the output between the intervention and covariate, and how do I interpret this?

    And on a more basic level: Is this the correct way of analysing the data? I also thought about using difference scores (Baseline – post 1 and Baseline – Post 2), and then compare these two difference scores in a repeated measures ANOVA: I don’t know what’s the bettwe way to do it: Use the baseline assessment as covariate or use difference scores?

    Thank you very much for your help! I am lost!! 🙂

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