The bar operator generates all possible interaction effects. When your model contains many interaction effects, you might be able to code them more parsimoniously by using the bar operator ( | ). In the preceding matrix, main-effects columns are not linearly independent of crossed-effects columns in fact, the column space for the crossed effects contains the space of the main effect. Table 56.17 Example of Interaction Effects Empty columns (that would contain all 0s) are not generated for, but they are for. The order of the columns is such that the rightmost variables in the cross index faster than the leftmost variables ( Table 56.17). Then, PROC MIXED generates columns for all combinations of levels that occur in the data. Thus, B* A becomes A* B if A precedes B in the CLASS statement.
With an interaction, PROC MIXED first reorders the terms to correspond to the order of the variables in the CLASS statement. Often a model includes interaction (crossed) effects.
