Interactions in factorial design

[DavidQ_R]

Hi - I can specify a full factorial model with factors A, B, C and D in PROC GLM, PROC GLMSELECT etc using notation like this: A | B | C | D.  But can I tell the procedure that I only want to include interactions of A, B, C and D upto and including the second order?  Obviously I could write them all out in full (A*B  A*C A*D B*C etc), but I'm guessing there's probably an easier way.  Is there?

Many thanks.

[SteveDenham · Accepted solution]

There sure is.  For your situation, use A|B|C|D@2.  This will give a model with main effects and all two way interactions.  In the documenation for GLM, look at Example 42.11 Analysis of a Screening Design, where they present a four factor model, and fit main effects and the two way interactions.

Steve Denham

[DavidQ_R]

Perfect - just what I needed.

Many thanks!