Hi Everyone, 

Sorry to ask what I think is a basic question but I really haven't found my answer yet in the archives. 

I am trying to run a mixed effects model in R using the lme package. My experiment is such that I am interested in the effects of Temperature (2 levels) and Species (3 levels) on Growth. I collected individuals from three populations within each species. Because individuals within a population are potentially more similar to each other than individuals among populations, I want to include population as a random factor in my model. 

I would have thought that I would structure the model as follows: 

z<-lme(Growth~Temp*Species, random=~1|Species/Population) 

But the summary for this model includes NAs (e.g. for two of the species). 

I've considered a model such as 

z<--lme(Growth~Temp*Species,random=~1|Population) 

but it strikes me that I need to associate/nest population and species (as not all the Species "treatments" are applicable to every population). 

I'm also confused as to the naming of populations. Currently I've got the populations named 1,2,3,4,5 ...9. Should I be naming them A1,A2,A3, B1,B2,B3, C1,C2, C3 (where the letters represent different species)? When does that matter? 

Any help with this would be greatly appreciated! 

	[[alternative HTML version deleted]]

