[R] Difference in Kaplan-Meier estimates plus CI

Terry Therneau therneau at mayo.edu
Thu Mar 8 15:08:10 CET 2012

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I thought this would be trivial, but I can't find a package or function
that does this.

I'm hoping someone can guide me to one.

Imagine a simple case with two survival curves (e.g. treatment &

I just want to calculate the difference in KM estimates at a specific
point (e.g. 1 year) plus the estimate's 95% CI.  The former is
straightforward, but the estimates not so much.

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As others have pointed out, use "survfit".  Because the curves are step
functions, they likely won't have a stored value in the data at exactly
365 days.  The summary() function for survival curves allows you to read
off the curve's values at any arbitrary time point.

A note on standard errors:  S(t) +- std is a terrible confidence
interval.  You will be much more accurate if you use log scale.  (Some
argue for logit or log-log, in truth they work well.)   If n is large
enough, however, you should be ok.

Terry T.

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