I am reading Applied Longitudinal Data Analysis by Fitzmaurice. However, I am confused as when to use mixed effect model.
The book says in general when we want to study impact on individuals of the study, we apply mixed and when we want to study population, we apply marginal.
However, I could not conjure up a situation in which I would like to study impact of treatment on individuals of the study in general. All I can come up is always why treatment should be applied and this is basically asking for populational level question.
What question in real world is going to make me choose mixed effect rather than marginal model? I am not concerned about generalizability of model from GLM provided by mixed effect GLM here.
