Statalist


[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: st: Re: Cox regression


From   Janet Hill <janethill73@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Re: Cox regression
Date   Mon, 2 Feb 2009 09:59:09 +0000 (GMT)

Steve,
Thanks for that, I thought that might be the case. 
I will look into exact tests. I think that the
distribution of the censoring times will preclude the
use of Fisher.
Janet
--- Steven Samuels <sjhsamuels@earthlink.net> wrote:

> Janet, You cannot fit a hazard model to a group with
> no observed  
> failures.  However your main problem is that you
> have too few  
> failures to trust the Cox results.  The p-values
> from the Cox mode  
> require larger sample sizes.  Plot the survival
> curves with the  
> asymptotic CI's (-sts graph-) and see how wide they
> are.  Your only  
> recourse is to an "exact" test. At least one such
> exact long-rank  
> test has been published, but I don't have the
> reference offhand.   If  
> follow-up times are similar among the censored
> observations, or the  
> distributions of censoring times in the three groups
> are similar, you  
> can try pairwise Fisher's exact tests with failure
> as a binary outcome.
> 
> Steve
> 
> > I want to use a Cox proportional hazards model
> with 1
> > independent variable corresponding to drug types.
> > There are 3 drugs and for 1 of these there are no
> > failures.
> >
> > . xi: stcox i.fadh
> > i.fadh            _Ifadh_1-3          (naturally
> > coded; _Ifadh_1 omitted)
> >
> >          failure _d:  death
> >    analysis time _t:  dt
> > Cox regression -- no ties
> >
> > No. of subjects =           60
> > Number of obs   =        60
> > No. of failures =           15
> > Time at risk    =      1326.29
> >                                                   
> LR
> > chi2(2)      =      9.61
> > Log likelihood  =   -47.766052
> > Prob > chi2     =    0.0082
> >
> >
>
----------------------------------------------------------------------
> 
> > --------
> >           _t | Haz. Ratio   Std. Err.      z   
> P>|z|
> >    [95% Conf. Interval]
> > ------------- 
> >
>
+----------------------------------------------------------------
> >     _Ifadh_2 |   1.68e+09   8.79e+08    40.49  
> 0.000
> >    5.99e+08    4.68e+09
> >     _Ifadh_3 |   4.33e+09          .        .     
>  .
> >           .           .
> >
> > My question is - how do I handle the situation
> where
> > one of the groups has no failures, and how do I
> > calculate a meaningful hazard ratio.
> 
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
> 



      
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



© Copyright 1996–2014 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index