[R] clogit question

vito muggeo vmuggeo at dssm.unipa.it
Thu Mar 23 16:38:01 CET 2006

Dear Dan,
I think you need more (theorical) background here..

clogit() in package survival performs conditional logistic regression 
where you have several groups (the strata, the matched sets). There is 
an intercept for each "stratum" in the model, but you do not obtain them 
since estimation is carried out via conditional likelihood, i.e. *given* 
the sufficient statistics for the intercepts themselves.

Have a look to standard book on categorical data analysis.

If you need estimates for the intercepts (try to) use glm():


Dan Chan wrote:
> Hi,
> I am playing with 
>      clogit(case~spontaneous+induced+strata(stratum),data=infert)
> from clogit help file.
> This line works. 
> 1. But, why strata(stratum) doesn't have a coefficient like spontaneous
> and induced?
> 2. When I remove strata(stratum) from the command, this function seems
> to keep running forever.  Why?  
> 3. I think the equation for clogit looks like
> P=1/(1+ exp(-1*(a+bx+cy+.....))
> In this example, I think the spontaneous is x, induced is y.  So, b is
> the coefficient for spontaneous and c is coefficient for induced.  Where
> can I find a?  
> Thank you. 
> Daniel Chan
> Meteorologist
> Georgia Forestry Commission
> P O Box 819
> Macon, GA 
> 31202
> Tel: 478-751-3508
> Fax: 478-751-3465
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Vito M.R. Muggeo
Dip.to Sc Statist e Matem `Vianelli'
Università di Palermo
viale delle Scienze, edificio 13
90128 Palermo - ITALY
tel: 091 6626240
fax: 091 485726/485612

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