[R] Error in GEE model fit

[david dav]

Hi,
I would like to fit a GLM model with GEE on clustered data.
I tried to use gee in the GEE package on a twin data set. All cluster
are of size 2. I removed the missing data and ordered by IDENTIF2 first.

library(gee)
mod.pc <- gee(Y ~ X1 + X2 , id = IDENTIF2,  family = binomial, corstr
= "unstructured", data = na.omit(df))

gives the following result :
Beginning Cgee S-function, @(#) geeformula.q 4.13 98/01/27
running glm to get initial regression estimate
(Intercept)         X1M  X2Sans AMP      X2Stim
 -2.7756284   0.4658861  -1.6508288  -1.5059518
Erreur dans gee(Y ~ X1 + X2, id = IDENTIF2, family = binomial, corstr
= "unstructured",  :
 NA/NaN/Inf dans un appel à une fonction externe (argument 2)   #
meaning in a call to an external function
De plus : Warning message:
In gee(Y ~ X1 + X2, id = IDENTIF2, family = binomial, corstr =
"unstructured",  :
 NAs introduits lors de la conversion automatique   # meaning NAs
introduced during the automatic conversion

The regression gives results
with SAS version 8 on the same dataset (results are pretty close from
the results of a naive logistic regression).
I tried to change the explanatory variables (X1 alone, X2 alone, other
possible explanatory variables : no result).
I do not find where the error comes from. Should I trust the results from SAS ?

Thank you for your help.
David
Using R2.7.2 and gee version 4.13-13

PS Naive logistic regression give similar results on R and SAS in my case