[R] Random and fixed effect model with a covariate
Sophie Alice Octavia Armitage
SAOArmitage at bi.ku.dk
Tue Jan 29 17:04:49 CET 2008
Dear All,
I wonder if anyone can please offer any advice on a model including 2 fixed effects and 1 random effect, as well as a covariate?
The experimental design is as follows:
I have a two by two factor design, where the two factors, Age (A) and Group size (G), both have 2 levels (old or young, and 1 or 3 respectively), and I am interested in the effect of these factors upon a continuous Response variable (R).
The data is from social insects, so the above design is repeated for several colonies. I think that Colony (C) should be a random factor, as they were taken from a larger pool of available colonies. Identification number (I) is different for each individual.
Furthermore, I have a covariate, Mass (M), which could have an effect upon R.
Do you know which of the below (if any) would be the most appropriate model please?
model<-lme(R~A*G*M,random=~1|C/I)
model<-lme(R~A*G+M,random=~1|C/I)
or is it necessary to state that A and G are factors as below:
model<-lme(R~as.factor(A)*as.factor(G)*M,random= ~1|C/I)
model<-lme(R~as.factor(A)*as.factor(G)+M,random= ~1|C/I)
Do you also please have any advice upon the effect it would have to change the order in which the factors and covariate are placed in the model? When I tried moving them around in the above models it changed the test statistics slightly.
Many thanks in advance for any help you can give, it’s very much appreciated.
Sophie Armitage
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