Thanks for your reply.

> So, COV varies across the combinations among the three factors W1-3 for each subject?

Yes, COV varies for the two levels of W1 (visit 1 and visit 2). This means that COV is the same for the levels of W2 and W3 per level of W1 (visit).

> corSymm() is used to specify the variance-covariance structure in the residuals. I would not worry too much about the residuals. Instead, it might make sense to try
-ranEff “~1+COV”

I'm afraid that I'm still a bit puzzled. Why should we not worry about the var-cov structure of the residuals? And what are we actually specifying if we use ranEff “~1+COV”?
We would like to adequately specify our repeated measures across the within subject variables W1 - W3. We see a big difference between the default settings, and specifying a structure under corStr, and also noticed that no matter what we specify under corStr, it results in the use of AR1 in the code, but we might be wrong. We think an unstructured matrix suits our data the best, but we are not sure how to set this up in 3dLME (but we know how to do it using R's lme). Any additional help/info would be highly appreciated.

Thanks,

Best
Hans