[R] Survival-Analysis: How to get numerical values from survfit (and not just a plot)?

Eik Vettorazzi E.Vettorazzi at uke.uni-hamburg.de
Tue Feb 17 17:44:06 CET 2009

Hi Bernhard,
I'm wondering what you will expect to get in "dividing" two proportional 
survival curves from a fitted cox model.
Anyway, you can provide a newdata object to the survfit function 
containing any combination of cofactors you are interested in and then 
use summary, eg:

fit <- coxph( Surv(futime,fustat)~resid.ds+rx+ecog.ps,data=ovarian)
summary(survfit( fit,newdata=data.frame(rx=1, ecog.ps=2, resid.ds=0)))


Bernhard Reinhardt schrieb:
> Hi!
> I came across R just a few days ago since I was looking for a toolbox 
> for cox-regression.
> I´ve read
> "Cox Proportional-Hazards Regression for Survival Data
> Appendix to An R and S-PLUS Companion to Applied Regression" from John 
> Fox.
> As described therein plotting survival-functions works well 
> (plot(survfit(model))). But I´d like to do some manipulation with the 
> survival-functions before plotting them e.g. dividing one 
> survival-function by another.
> survfit() only returns an object of the following structure
>      n events median 0.9LCL 0.9UCL
> 55.000 55.000  1.033  0.696  1.637
> Can you tell me how I can calculate a survival- or baseline-function 
> out of these values and how I extract the values from the object? I´m 
> sure the calculation is done by the corresponding plot-routine, but I 
> couldn´t find that one either.
> Regards
> Bernhard
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Eik Vettorazzi
Institut für Medizinische Biometrie und Epidemiologie
Universitätsklinikum Hamburg-Eppendorf

Martinistr. 52
20246 Hamburg

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