[R-sig-ME] Rasch with lme4

[Daniel Ezra Johnson]

> 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