[R] Kaplan Meier - not for dates

Terry Therneau therneau at mayo.edu
Mon Nov 7 14:06:10 CET 2011

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I have some data which is censored and I want to determine the median.
Its actually cost data for a cohort of patients, many of whom are still
on treatment and so are censored.

I can do the same sort of analysis for a survival curve and get the
median survival... ...but can I just use the survival curve functions to
plot an X axis that is $ rather than date? If not is there some other
way to achieve this?
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 1. The survfit routines will work, and the results that you plot will
indeed be on the dollar scale, BUT

  2. The answer will be wrong.  The reason is that the censoring occurs
on a time scale, not a $ scale: you don't stop observing someone because
total cost hits a threshold, but because calendar time does.  The KM
routines assume that the censoring process and the event process are on
the same scale.
  The result can be an overestimation of cost.  See Dan-Yu Lin,
Biometrics 1997, "Estimating medical costs from incomplete follow-up

Terry Therneau

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