Run the model with HPSU as  fixed-effect and check the F-value for HPSU,   Some good advice found in Tips and Strategies for Mixed Modeling with SAS/STAT® Procedures http://support.sas.com/resources/papers/proceedings12/332-2012.pdf and  Usage Note 40724: Comparing covariance structures, testing covariance parameters using the COVTEST statement in PROC GLIMMIX http://support.sas.com/kb/40/724.html ... View more