Specifying Effects in the MODEL Statement

The FACTEX procedure accepts models that contain terms for main effects and interactions. Main effects are specified by writing variable names by themselves.

Interactions are specified by joining variable names with asterisks.

In addition, the bar operator (|) simplifies specification for interactions. The @ operator, used in combination with the bar operator, further simplifies specification of interactions. For example, two ways of writing the complete set of effects for a model with up to three-factor interactions are

model estimate=(A B C A*B A*C B*C A*B*C);

and

model estimate=(A|B|C);

When the bar (|) is used, the right- and left-hand sides become effects, and their cross becomes an interaction effect. Multiple bars are permitted. The expressions are expanded from left to right, using rules given by Searle (1971). For example, A|B|C is evaluated as follows:

You can also specify the maximum number of variables involved in any effect that results from bar evaluation by specifying the number, preceded by an @ sign, at the end of the bar effect. For example, the specification A|B|C@2 results in only those effects that contain two or fewer factors. In this case, the effects A, B, A*B, C, A*C, and B*C are generated.