Missing Level Combinations

PROC GLIMMIX handles missing level combinations of classification variables in the same manner as PROC GLM and PROC MIXED. These procedures delete fixed-effects parameters corresponding to missing levels in order to preserve estimability. However, PROC GLIMMIX does not delete missing level combinations for random-effects parameters because linear combinations of the random-effects parameters are always predictable. These conventions can affect the way you specify your CONTRAST and ESTIMATE coefficients.