I have longitudinal data following patients before and after a dichotomous treatment. Exploratory analyses strongly suggest that the effect of the treatment depends on the initial burden of the patient: If a patient's burden is relatively low the treatment won't help the patient much, but if the initial burden of the patient is high, the treatment will help a lot.
How can I analyze this using inferential statistics? How can I estimate the treatment effect depending on the initial burden of the patient, and obtain its standard error?
I initially thought about seperating the patients in groups based on the intial burden and then to run a simple moderator analysis. This procedure seems however very crude to me. Isn't there some more appropriate technique?