Proc mixed interaction term producing strange output

[Dimi]

Hi Everyone, 

Thanks for taking the time to read this. 

I'm having a bit of trouble with PROC MIXED -namely when modeling an interaction term between a continuous and binary variable. 

The model part of my code reads like this: 

model result = arm_studyall*visit_m arm_studyall visit_m /solution;

 For clarity result is a continuous variable. arm_studyall is a binary variable and visit_m is a continuous variable.

When I execute my code the fixed part of the model produces an interaction term for both levels of the binary variable



I'm not quite sure why this is happening -as I would expect only one interaction term to be estimated. 

Any insight would be helpful. 

Thanks again. 

[PaigeMiller]

You get an effect (slope) of visit_m when arm_studyall is zero.

You also get an effect (slope) of visit_m when arm_studyall is one.

This is the exact meaning of an interaction between continuous and binary variable ... there are really two different slopes.

[FreelanceReinh]

Hi @Dimi,

If you want only one of the interaction terms to have a nonzero estimate, just reorder the effects in your model statement, so that main effects are prioritized:

model result = arm_studyall visit_m arm_studyall*visit_m /solution;