Hi, I am wondering how the confidence interval for Kaplan-Meier estimator is calculated by survfit(). For example, 

> summary(survfit(Surv(time,status)~1,data),times=10)
Call: survfit(formula = Surv(rtime10, rstat10) ~ 1, data = mgi)

 time n.risk n.event survival std.err lower 95% CI upper 95% CI
   10    168      55    0.761  0.0282        0.707        0.818


I am trying to reproduce the upper and lower CI by using standard error. As far I understand, the default method for survfit() to calculate confidence interval is on the log survival scale, so:


upper CI = exp(log(0.761)+qnorm(0.975)*0.0282) = 0.804
lower CI = exp(log(0.761)-qnorm(0.975)*0.0282) = 0.720


they are not the same as the output from survfit().

Am I missing something?

Thanks

John

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