I have a longitudinal dataset where I'm building an lmer model using 5 fixed predictors with a random effect for subjects. One of the fixed predictors is Diagnosis ( there are three different diagnosis and I'm interested in seeing the change in outcome for each diagnosis)
I built a full model, but didn't add any interaction terms because I don't know how to go about it. Do I run models with all 16 two way interactions between the fixed predictors and then check for significance by using anova() and comparing to my full model for each interaction? Do I then check for 3-way interactions, 4-way interactions and so on?
N.B- I'm new to all this with no statistical background