> Dear all,
> I am confused with the output of survfit.coxph.
> Someone said that the survival given by summary(survfit.coxph) is the
> baseline survival S_0, but some said that is the survival
> S=S_0^exp{beta*x}.
>
> Which one is correct?

 The ³baseline survival², which is the survival for a hypothetical subject
with all covariates=0, may be useful mathematical shorthand when writing a
book but I cannot think of a single case where the resulting curve would be
of any practical interest in medical data.  For this reason my survival
routines in R NEVER return it.  (Ask yourself ³what is the survival for
someone with blood pressure=0, cholesterol=0, weight=0, ....².  The answer
is that they are either non-existent or dead).
 The intention with survfit is that you will give it a second data set
containing one or more lines, each of which describes a subject whose
predicted survival is of interest.  If no such data is given, the survival
for someone with all covariates = to the mean is given.  This is better than
covariates =0, but sometimes not by much.  (What if sex were coded as a 0/1
numeric ‹ do we get the survival of a hermaphrodite?)

Your best approach is to forget the phrase ³baseline survival² and focus on
covariate sets of interest to you.

Terry Therneau

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