R help - May 2004 - multiple nesting levels in GEE

Hello,

I'm actually trying to fit a gee model with 2 nesting levels since I expect
a
correlation between all members of a litter at a first level and between all 
individuals sharing a mother at a second superior level with an exchangeable 
matrix. I order my dataframe by both mother and litter

I try several syntaxes:
id= mother*litter which give the same correlation matrix as id= litter*mother
and id=litter with a matrix size of 7*7 which correspond to the maximum number
of
young per litter

id=litter|mother which give the same correlation matrix as id=mother|litter with
a correlation matrix size of 235*235 which correspond to the number of 
observations

id=litter/mother and id=mother/litter give different result with a strange
matrix
of 11*11 with no 1 on the diagonal

If anybody know if it is possible to construct nesting levels in R and what is 
the good syntax for this kind of model?

Thanks in advance


Aur??lie Cohas

On Sat, 29 May 2004 Aurelie.Cohas at univ-lyon1.fr wrote:
>  Hello,
>
> I'm actually trying to fit a gee model with 2 nesting levels since I
expect a
> correlation between all members of a litter at a first level and between
all
> individuals sharing a mother at a second superior level with an
exchangeable
> matrix. I order my dataframe by both mother and litter
The gee() function does not compute this correlation structure (because
no-one has implemented it).  I don't think other software implements it
either, although people keep pointing out that it would be useful.

  This need not matter -- the point of GEE is that the correlation
structure is not important for validity of inference (although it may
affect efficiency).  If you are fitting a linear model I would recommend
lme(), otherwise you should just be able to use an exchangeable model at
the largest clustering size.

	-thomas

> I try several syntaxes:
> id= mother*litter which give the same correlation matrix as id=
litter*mother
> and id=litter with a matrix size of 7*7 which correspond to the maximum
number of
> young per litter
>
> id=litter|mother which give the same correlation matrix as id=mother|litter
with
> a correlation matrix size of 235*235 which correspond to the number of
> observations
>
> id=litter/mother and id=mother/litter give different result with a strange
matrix
> of 11*11 with no 1 on the diagonal
>
> If anybody know if it is possible to construct nesting levels in R and what
is
> the good syntax for this kind of model?
>
> Thanks in advance
>
>
> Aur??lie Cohas
>
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Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle