[R] Can nonlinear models be used in gee?

[Jinn-Yuh Guh]

Hi all,
1. Can nonlinear models be used in gee? For example, I have a dataset which contains 2 variables x and y, I wrote
data(ex)
atttach(ex)
a<-100
b<- -0.5
c<-4.5
d<-20
Then:
a. y~gee(y~d+(a-d)/(1+(x/c)^b))
Error in terms.formula(formula, data = data) : 
        invalid power in formula
b. y~gee(y~d+(a-d)/(1+(x/c))
Error in model.frame(formula, rownames, variables, varnames, extras, extranames,  : 
        variable lengths differ
Why?

Thanks!
Sincerely Yours,
Jinn-Yuh Guh, M.D.
Dept. of Internal Medicine
Kaohsiung Medical College
100 Shi-Chuan 1st Road
Kaohsiung, Taiwan
FAX: 886-7-312-2810
e-mail: jyuh at mail.nsysu.edu.tw

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