[R-sig-ME] Rasch with lme4

Daniel Ezra Johnson danielezrajohnson at gmail.com
Tue Jun 9 16:09:36 CEST 2009

> 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.


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