[R] survival package - calculating probability to survive a given time

[David Winsemius]

On Dec 10, 2010, at 2:07 PM, Andreas Wittmann wrote:

> Dear R users,
>
> i try to calculate the probabilty to survive a given time by using  
> the estimated survival curve by kaplan meier.
>
> What is the right way to do that? as far as is see i cannot use the  
> predict-methods from the survival package?
>
> library(survival)
> set.seed(1)
> time <- cumsum(rexp(1000)/10)
> status <- rbinom(1000, 1, 0.5)
>
> ## kaplan meier estimates
> fit <- survfit(Surv(time, status) ~ 1)
> s <- summary(fit)
>
> ## 1. possibility to get the probability for surviving 20 units of  
> time
> ind <- findInterval(20, s$time)
> cbind(s$surv[ind], s$time[ind])

See if this helps:

 > head(which(s$surv < 0.5))
[1] 368 369 370 371 372 373
 > plot(fit)
 > abline(h=0.5)
 > abline(v=s$time[368])


>
> ## 2. possibility to get the probability for surviving 20 units of  
> time
> ind <- s$time >= 20
> sum(ind) / length(ind)


-- 
David Winsemius, MD
West Hartford, CT