[R-sig-ME] RE : Interactions between fixed and random effects

[Julien Beguin]

If you have 22 populations, and assuming that each indivudual in each population is randomly distributed within you water treatment (you did not specify your experimenta design... so I guess), you might have enough power to test the interaction between plant population and water treatment (21 df) in a similar way than in a classic ANOVA. So why to specify plant population as a random variable, rather than a fixed factor? are you (not) intersted to know which plant population respond differently regarding watering regimes? 

proposition:

> lm(DryMass~Water*Population) 


just a thought...

Julien Beguin  

________________________________________
De : r-sig-mixed-models-bounces at r-project.org [r-sig-mixed-models-bounces at r-project.org] de la part de Christopher Eckert [chris.eckert at queensu.ca]
Date d'envoi : 4 avril 2010 23:39
À : r-sig-mixed-models at r-project.org
Objet : [R-sig-ME] Interactions between fixed and random effects

I apologize if the answer to this query is somewhere totally obvious, but i couldn't find it.

I am trying to analyze an experiment where a set of 22 populations of a dune plant species (populations were randomly chosen from across the species' geographic range) were grown in a glasshouse under two different watering regimes (Water = Control vs. Drought). DryMass is the response variable. There was about 20 individuals from each population grown in each Water treatment.

Population is a random effect, but I would like to test for an interaction between Population and Water -to ask the question: do different populations respond differently to drought?

>From what I can gather this is analogous the random intercepts and slopes model discussed in Zuur et al and elsewhere, except that I am examining a categorical predictor (Water) rather than a continuous predictor.

Am I right in thinking that the basic syntax using lme is:

lme(DryMass~Water,random=~Water|Population)

and the syntax using lmer is:

lmer(DryMass~Water+(Water|Population))

Thanks very much for any clarification you can send my way.

Chris Eckert
Department of Biology
Queen's University
Kingston, Ontario K7L 3N6 Canada
chris.eckert at queensu.ca

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