Message-ID: <a46630750906090709w223b7a80hfe45ee4901d30bc@mail.gmail.com>
Date: 2009-06-09T14:09:36Z
From: Daniel Ezra Johnson
Subject: Rasch with lme4
In-Reply-To: <4A2E4083.6060204@sbg.ac.at>
> 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