Skew-normal Linear Mixed Models

Abstract

Normality (symmetric) of the random effects and the within subject errors is a routine assumptions for the linear mixed model, but it may be unrealistic, obscuring important features of among- and within-subjects variation. We relax this assumption by considering that the random effects and model errors follow a skew-normal distributions, which includes normal ity as a special case and provides flexibility in capturing a broad range of non-normal behavior. The marginal distribution for the observed quantity is derived which is expressed in closed form, so inference may be carried out using existing statistical software and standard optimization techniques. We also implement an EM type algorithm which seem to provide some ad vantages over a direct maximization of the likelihood. Results of simulation studies and applications to real data sets are reported.