The default technique is METHOD=RSPL, corresponding to maximizing the residual log pseudo-likelihood with an expansion about the current solutions of the best linear unbiased predictors of the random effects. In models for normal data with identity link, METHOD=RSPL and METHOD=RMPL are equivalent to restricted maximum likelihood estimation, and METHOD=MSPL and METHOD=MMPL are equivalent to maximum likelihood estimation.
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The following SAS Usage Note:
http://support.sas.com/kb/37107
http://support.sas.com/kb/40724
provide information on testing covariance parameters when using PROC MIXED and PROC GLIMMIX.