# Re: st: Re: Cox regression

 From Steven Samuels To statalist@hsphsun2.harvard.edu Subject Re: st: Re: Cox regression Date Sun, 1 Feb 2009 13:39:24 -0500

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.

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/
```