# Re: st: Re: Cox regression

 From Janet Hill 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

> 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
> 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
> >
> >          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.
>
> *
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>

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```