[R] Basic LME

Martin Hoyle
Tue, 08 Apr 2003 14:08:32 -0000

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.

So, my questions are;

1: I expected that the LME model would be a better way to analyse this data
compared to the AOV model, since Genotype is a random factor. However, I obtain
the same parameter estimate and p value for Age. Please can someone tell me why?

2: When using LME, when I am after a p value for the covariate Age, is it better
to do the following;

Model.lme2 <- lme (Weight~Age, random=~ 1|Genotype, method="ML")
Model.lme3 <- lme (Weight~1, random=~ 1|Genotype, method="ML")
Anova(Model.lme2, Model.lme3)

Giving likelihood ratio=0.102, with p=0.749, which is slightly different to the
p values of 0.758 above.


Thanks for your attention,
Martin.


Martin Hoyle,
School of Life and Environmental Sciences,
University of Nottingham,
University Park,
Nottingham,
NG7 2RD,
UK
Webpage: http://myprofile.cos.com/martinhoyle