[R-sig-ME] random effects specification

Sebastian P. Luque spluque at gmail.com
Fri Apr 4 15:05:10 CEST 2008

On Fri, 4 Apr 2008 07:17:36 -0500,
"Douglas Bates" <bates at stat.wisc.edu> wrote:


> I'm not sure that I understand what you mean by "treatment being
> nested within community".  Does this mean that there are really 8
> different treatments because treatment 1 in community A is different
> from treatment 1 in community B?  If so, then it would make sense to
> me to simply create a new factor that is the interaction of treatment
> and community.

I was not employing the term "nested" properly.  The number of levels
for both community and treatment are 2 and 4, respectively, just as in
the example.  The same 4 treatments were used in both communties, so in
fact, treatment is crossed with community, not nested.  However,
subjects are nested within communities because each subject belongs to
one community only, yet received all 4 treatments.  Sorry for this

> Perhaps I am approaching the community factor incorrectly.  In your
> data there are two communities so, even if it would be reasonable to
> model community effects as random effects, that would be difficult.
> With only two levels I think it is best modeled as a fixed effect,
> which would mean that questions about treatment and community are
> related to the fixed effects.

Could you please show a formula for the case where each individual is
seen at both communities (community and treatment still being fixed)?
This would help me understand the syntax better.

Thank you so much for your help.


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