Hi there,

I am trying to simulate mixed model data for a one-way within subjects
design with three treatment conditions to compare the power of RM-ANOVA and
mixed models.

One major goal is to be able to manipulate compound symmetry to be either
violated or met.

Currently, I am sampling one random intercept and one random slope for each
subject and add it to the different groups via sum contrasts eg:

random slope for subject i = 10 and random intercept = 40, then:

Subj i in treat1 -> 40 + 10
Subj i in treat2 -> 40 + 10
Subj i in treat 3 -> 40 - 20

This leads to perfect positive (negative) correlations between random
slopes so compound symmetry will be violated. Even though the resulting DV
will also be influenced by the random intercept and the residuals, this is
still the case. But, how is it possible to NOT violate CS in such a case? I
could somehow try to sample correlated residuals for each treatment
conditions but this does not seem to be the best way.

Another approach would be to sample one random slope for each treatment
condition and subject. Then, I could either sample with different
correlations (CS violated) or equal correlations (CS met). But this would
lead to the random slopes not to sum to zero for each subject.

Do you have any suggestions on this?

Thanks in advance

Regards,
Ulf

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