[R] Specifying an lme model

[Ross Darnell]

I would like some advice on how if possible, to test the following

 I have subjects each measured several times. The subjects are sampled
 from 3 subpopulations (groups). The question is "Is the 
 between-subject variance the same for the three groups?"

The "null" model is 

lme0 <- lme(y~group,random=~1|subject)

I did think that the model that defined a specific between-subject
variance for each group was

update(lme0,.~., weights=varIdent(form=~1|group))

but I am not sure.

I would appreciate any help.

Ross Darnell