I am trying to fit a linear mixed effects model with several fixed effects and a random intercept that varies per subject.
My problem is that I know that one of the fixed variables, let's call it 'A', depends on another one, 'B', which is categorical. I want to search for associations of 'A' with the response variable, taking into account that 'B' can affect 'A'.
How should I model this? Should I introduce an interaction term, 'A*B' in my model? Or should I introduce a random effect like : (-1 + A | B)? In either of the cases, which should be the association of 'A' and the response variable, how should I identify it?
Thank you very much for your time and effort,