[R] Multiple random effects inlme?

[Anon.]

Moi!

I was helping to teach a course on mixed models this week, and we came
across a problem with coding more than one random effect in lme when
they aren't nested.

As an example, suppose we have an experiment where we sample moths from
several populations, and place the moths on different trees, and measure
a trait (in this case survival of offspring, but that's less
important).  We want to treat Population and Tree as random effects, as
that will tell us something about adaptation.  Naively, i would expect
to code the data in lme as something like:

lme(x ~ 1, random=~1|(Pop + Tree))

but this doesn't work, and I can't work see what to do (or where to look
to find out!).  Can anyone point me in the right direction?

Thanks in advance.

Bob

-- 
Bob O'Hara

Rolf Nevanlinna Institute
P.O. Box 4 (Yliopistonkatu 5)
FIN-00014 University of Helsinki
Finland
Telephone: +358-9-191 23743 
Mobile: +358 50 599 0540
Fax:  +358-9-191 22 779
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