R help - Dec 2008 - gee + rcs

Hi all,
 
I have fitted a gee model with the gee package and included restricted cubic
spline functions. Here is the model:
 
chol.g <- gee(SKIN ~ rcs(CHOLT, 3), id=ID, data=chol,
family=binomial(link="logit"), corstr="exchangeable")

To extract the log odds I use:
 
predict.glm(chol.g, type = "link")
 
Now I want to compute the logg odds for specific CHOLT values (the dependent
variable) that I want to choose myself (i.e. that are not available in the
dataset). Is there a way to get the linear predictor of the gee model including
all separate spline functions and related coefficients? Latex from the Design
package does not work here.
 
Thanks for your help!
 
kind regards, 
 
Martijn W Heymans

Faculty of Earth and Life Sciences

Institute of Health Sciences

Department of Methodology and Applied Biostatistics
VU University 

Amsterdam, the Netherlands

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Heymans, M.W. wrote:
> Hi all,
>  
> I have fitted a gee model with the gee package and included restricted
cubic spline functions. Here is the model:
>  
> chol.g <- gee(SKIN ~ rcs(CHOLT, 3), id=ID, data=chol,
family=binomial(link="logit"), corstr="exchangeable")
> 
> To extract the log odds I use:
>  
> predict.glm(chol.g, type = "link")
I wonder if such predictions are 'safe', i.e., use the original knot 
locations hidden in an attribute by rcs.
>  
> Now I want to compute the logg odds for specific CHOLT values (the
dependent variable) that I want to choose myself (i.e. that are not available in
the dataset). Is there a way to get the linear predictor of the gee model
including all separate spline functions and related coefficients? Latex from the
Design package does not work here.
You'll either need to write a wrapper function for gee like glmD is for 
glm, or use lrm in Design with intra-cluster correlation correction 
using robcov or bootcov.  However this would assume a working 
independence model rather than a compound symmetry working model.

Frank
>  
> Thanks for your help!
>  
> kind regards, 
>  
> Martijn W Heymans
> 
> Faculty of Earth and Life Sciences
> 
> Institute of Health Sciences
> 
> Department of Methodology and Applied Biostatistics
> VU University 
> 
> Amsterdam, the Netherlands
> 
> 	[[alternative HTML version deleted]]
> 
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-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University