Rasch with lme4
What happens in practice when you compare the two approaches of [Subject] as a fixed versus as a random effect? Consider: M1 = lmer(Reaction ~ Days + (1|Subject), sleepstudy) M2 = lm(Reaction ~ Days + factor(Subject), sleepstudy) The slope estimates for Days for are practically identical, the mean intercepts differ...
One important difference arises if you have a fixed effect that, unlike Days, is "between subject", like Gender for example. In that case you MUST use the M1 (mixed-model) approach with Subject as a random effect. You CANNOT have Gender as a fixed effect AND Subject as a fixed effect. Dan