R help - Apr 2000 - Can nonlinear models be used in gee?

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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On Tue, 4 Apr 2000, Jinn-Yuh Guh wrote:
> 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
No, they can't. However, if you are using family=gaussian(), as you seem
to be, you could fit the model with nlme() instead.

	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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On Tue, 4 Apr 2000, Jinn-Yuh Guh wrote:
> 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?
Not in the gee function in addon package gee. Try the help!  It points you
to lm.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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