Mixed linear models are a particular class of models containing both fixed and random effects. Loosely speaking, a mixed model is a model where some terms remain constant over repeated sampling and some other terms vary at random according to some distribution. For simplicity from now on we will drop the notation linear and refer to these models simply as mixed models. For any mixed model we can identify three main components: the equation of the model, the expectations and Variance-Covariance for the random effects, and all the remaining assumptions regarding the model.