I'm trying to overcome the problem of related errors due to repeated measurements by using LMM instead of linear regression. However, SPSS mixed allows one to specify /RANDOM factors and/or /Repeated factors and I don't know which to use (or both).

In my situation, if I use both, the model does not converge. Using repeated only yields the best model (in terms of fit) but I'm not sure if only specifying the repeated factor is sufficient. Somewhere else I read in that case, it's actually a 'Marginal Model' (aka population-averaged model) and can be used if some requirements are met:

http://www.analysisfactor.com/statchat/repeated-measures-approaches/

However, I'm not sure if it's applicable to my data. I have four measurements on each subject, so I'm trying to model the lack of independence between observations. Can someone help me out? I have some books and articles on the use of this function in SPSS but they seem to contradict each other on this matter.