[R] [newbie] Cox Baseline Hazard

[Peter Dalgaard BSA]

Meles MELES <meles at free.fr> writes:

> And here is my first question.
> I'm actually trying to model a survival curve using a cox model.
> But I need the baseline cumulative hazard. I've been looking around on 
> the web and the archives, and I found that it as to do with the 
> nelson-aalen estimation.
> But I must not understand all the doc very well, because I can't get 
> the baseline hazard.
> Could you help me, please.

survfit(coxph(...)) gives you the baseline survival function, which by
default (type="aalen") is calculated as S(t) = exp(-Lambda(t))
where Lambda(t) is the cumulative hazard. So all you have to do is to
reverse the formula as Lambda(t) = -log(S(t))

The estimated S(t) is in the $surv component of the object returned by
survfit(). 

e.g.

example(survfit)
s <- survfit(fit)
plot(c(0,s$time),c(0,-log(s$surv)),type='s')

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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