Hello,
Your reference level for gender is '2' (Female).
Your reference level for age_grps is "12-17" .
If you want a single reference category for the interaction analysis, I would make the interaction effect upfront, like :
gender_times_age_grps = catx(" + ", gender , age_grps );
Then put that self-made interaction effect in the CLASS statement and specify the reference level you want (ref= ).
In the model statement, use the three variables specified in the CLASS statement and no other effects.
Thanks,
Koen