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 <[email protected]> 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.
>
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