The GLIMMIX Procedure

Interaction Effects

Often a model includes interaction (crossed) effects. With an interaction, PROC GLIMMIX first reorders the terms to correspond to the order of the variables in the CLASS statement. Thus, B*A becomes A*B if A precedes B in the CLASS statement. Then, PROC GLIMMIX generates columns for all combinations of levels that occur in the data. The order of the columns is such that the rightmost variables in the cross index faster than the leftmost variables. Empty columns (which would contain all 0s) are not generated for $\bX $, but they are for $\bZ $.

See Table 19.5 in the section GLM Parameterization of Classification Variables and Effects of Chapter 19: Shared Concepts and Topics, for an example of an interaction parameterization.