SAS testing interactions

[#1]

Hello

I am trying to test multiplicative interaction between 2 categorical variables with more than 2 categories. 

Would it be appropriate to use this code?

proc phreg data=new;

class diet_scorecategory (ref="1");

model (time*outcome(0) = bmicategory diet_scorecategory interactionterm;

interactionterm=bmicategory*diet_scorecategory;

run;

I would appreciate the help!

Thank you 

[FreelanceReinh · Accepted solution]

Hello @anonymous_user,

As a categorical variable bmicategory should be inserted into the CLASS statement. In many cases categorical variables are character variables, so a multiplication as in your assignment statement would be invalid anyway. Even if both categorical variables are numeric, interactionterm will not be recognized as an interaction term (despite its name and definition) -- with several degrees of freedom (depending on the numbers of categories and the parameterization method) --, but treated as a continuous variable (with only one degree of freedom), which is likely not what you want. Therefore, omit the assignment statement and replace interactionterm with bmicategory*diet_scorecategory in the MODEL statement.

[#3]

Hello @FreelanceReinh many thanks! It worked