I was recently told that it is not allowed to include static covariates in mixed effect models. I can believe that this could be true, however, when I have time series data, and season is a grouping factor, then the differences between the static covariates in the different seasons should be tested, or not?
If it is not allowed, how could the effect of the static covariates then be tested for? only with a repeated measures anova?
sorry for this confusing question.