Add the point for a starting time (time 0) to a survfit object's elements. This is useful for plotting.
survfit0(x, start.time=0)a survfit object
the desired starting time; see details below.
a reformulated version of the object with an initial data point
at start.time added.
The time, surv, pstate, cumhaz,
std.err, and std.cumhaz components will all be aliged,
so as to make plots and summaries easier to produce.
Survival curves are traditionally plotted forward from time 0, but
since the true starting time is not known as a part of the data,
the survfit routine does not include a time 0 value in
the resulting object.
Someone might look at cumulative mortgage defaults versus calendar
year, for instance, with the `time' value a Date object.
The plotted curve probably should not start at 0 = 1970/01/01.
(One could certainly argue that this behavior is overly pedantic, but
the decision was made over 20 years ago and it is far too late to
change it now.)
This is a utility function used by plot.survfit and
summary.survfit. If the original survfit call
included a start.time argument then that will be used as
the starting point, otherwise the default is 0.
It may be useful to others, so was exported.