Mixed Model Analysis of Data : Transition from GLM to MIXED

The GLM procedure has been the workhorse for mixed model applications in the SAS® System. Other procedures, including NESTED and VARCOMP, are used for specific applications. Since its introduction in 1976, GLM has been enhanced with several mixed model facilities such as the RANDOM and REPEATED statements. However, there are aspects of certain models that none of these facilities fully accommodate, such as structured covariance matrices for repeated measures data. The MIXED procedure allows applications of models previously not possible within the SAS System. In this paper, an overview comparison is given between GLM and MIXED. Examples will be presented comparing output from both procedures when either is appropriate to assist users in the transition from GLM to MIXED. Other examples will be presented which illustrate the use of MIXED in applications for which GLM is not adequate.