[R] Basic LME

[ritz@dina.kvl.dk]

Martin Hoyle <plxmh at nottingham.ac.uk> said:

> Hello R Users,
> I am investigating the basic use of the LME function, using the following 
example;
> 
> Response is Weight, covariate is Age, random factor is Genotype
> 
> model.lme <- lme (Weight~Age, random=~ 1|Genotype)
> 
> After summary(model.lme), I find that the estimate of Age is 0.098 with 
p=0.758.
> 
> I am comparing the above model with the AOV function;
> 
> model.aov <- aov (Weight~Age + Genotype)
> 
> I find that the estimate of Age is also 0.098, and p=0.758 as in the LME 
model above.

Have a look at summary(model.lme), find the "Random Effect:"-part and see 
whether the Genotype variation is close to 0. If so, this would explain why 
the estimates are similar, as the random factor Genotype then has almost 
negligible variation. 

Pinheiro, J. C. and Bates, D. M. (2000): "Mixed-Effects Models in S and S-
PLUS" discuss the merits of the different tests.

Christian