Are there any parametric assumptions for Linear Mixed Models - lmer (multivariate model)

Question

Are there any parametric assumptions for Linear Mixed Models - lmer (multivariate model)? I saw a few videos or journals that have reported results without resulting in any assumptions? any answer with reference will be appreciated

Answer 1

The fact that they didn't mention assumptions doesn't mean there aren't any. Many strong assumptions are made, e.g.,

• The right hand side of the model is structurally correct with regard to nonlinearities and interactions
• The overall error term is normal with constant variance
• The random effects are normal with constant variance
• The within-subject correlation is constant. If analyzing longitudinal data this is unlikely to be true, as typically correlations are stronger for measurements taken closer together in time.