# R code for CD4 seizure data (Diggle et al., 2002)
# VHM 882, Winter 2005, lecture 10L

cd4 <- read.table("e:/vhm/vhm882/data/cd4.dat",header=F,col.names=c("id","count","visit","tx","age","weeks"),row.names=NULL)

# gee (using gee library from gee package)
library(gee)
cd4.gee1 <- gee(count~tx*I(visit>0)+offset(log(weeks)), data=cd4, family=poisson, id=id, corstr="exchangeable")
summary(cd4.gee1)
cd4.gee2 <- gee(count~tx*I(visit>0)+offset(log(weeks)), data=cd4[cd4$id!=207,,], family=poisson, id=id, corstr="exchangeable")
summary(cd4.gee2)