[R] clogit question

[Dan Chan]

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