The Mixed Models task
fits a variety of mixed linear models to data and enables you to use
these fitted models to make inferences about the data. A mixed linear
model is a generalization of the standard linear model. The generalization
is that the data is permitted to exhibit correlation and nonconstant
variability. Therefore, the mixed linear model provides the flexibility
to model the means of your data (as in the standard linear model)
but also the variances and covariances.