Interaction effects of nominal and ordinal variables on binary dependant variable


I'm doing an analysis of interaction effects of one binary nominal variable (medication used), one ordinal variable (duration of therapy ; 1-few weeks, 2-less than 3 months, 4-less than six months, 5-more than six months) and one interval variable (age) on binary dependant variable (0- does not have a symptom, 1-does have a symptom).

I know i can use logistic regression to get individual effects of each independent variable on propability of having a symptom but I'm confused about how to test the interaction effects.

From what I know the regression analysis can be used for this purpose when the predictor (independant) variable is interval but can it be used in this case?? From what I understand when doing so the predictor variables have to be standardized but it doesn't make much sense to me to standardize nominal and ordinal variables.

What should I do?

Thank you!


Less is more. Stay pure. Stay poor.
I believe you can introduce an interaction term into the model (nominal*ordinal) and look for significance in the beta coefficients. I have little experience standardizing, but I agree that standardizing is usually withheld for continuous variables and some feel it is problematic with other types of variables.