*Details of the FACTEX Procedure* |

## 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.

`A B C`

*Interactions* are specified by joining variable names with
asterisks.
`A*B B*C A*B*C`

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.

Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.