Random & fixed effects on same level

Message: 2
Date: Wed, 20 Jan 2010 17:32:30 +0100
From: "De Smedt Sebastiaan" <Sebastiaan.DeSmedt at ua.ac.be>
To: <r-sig-mixed-models at r-project.org>
Subject: [R-sig-ME] Random & fixed effects on same level
Message-ID:
	<930B1A45F446404FA4D99A46F09209C401540E0A at xmail05.ad.ua.ac.be>
Content-Type: text/plain

Dear mailing list,





I have a model where I use leaf characteristics as response variables,
and populations and trees (nested in populations) as random effects.
After quantifying the variation associated with the different grouping
levels (error/within trees/within populations), I've added fixed
effects. I have information on population level (annual precipitation,
soil characteristics ...). By adding explanatory variables on population
level, can I still include 'population' as a random effect (without
breaking any statistical rule)? My idea was that, by including a fixed
effect, the fixed effect partially explains the variation added by a
grouping variable, but now I'm hesitating a bit... Does anybody have
some references on this topic?





Thanks in advance,

Sebastiaan