Hello,
I am learning about mixed-effects models and have a question. I am analyzing within-individual changes overtime in my analysis, but I don't know if I should included cases that do not have my key independent variable. Specifically, I am analyzing within-individual changes in grades among college students before and after they go to writing services. So my key independent variable is 'going to writing center' (0=No, 1=Yes) and my dependent variable are their grades. However, I don't know if I should include the rest of college students (and their grades) in my analysis, as these college students are not exposed to writing services. For what I have read, mixed-effects models should only include in the estimates the students who go to the writing center. I would greatly appreciate any assistance you could give me on this issue, and I would also appreciate if you could recommend me some literature about this topic. Thank you!