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, std.cumhaz and other components will all be aligned,
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.
Due to this uncertainty, it was decided not to include a "time 0" as
part of a survfit object. If the original survfit call
included a start.time argument, that value is of course
retained.
Whether that (1989) decision was wise or
foolish, it is now far too late to change it. (We tried it once as a
trial, resulting in over 20 errors in the survival test suite. We
extrapolate that it might break 1/2 - 2/3 of the other CRAN packages
that depend on survival, if made a default.)
If the original survfit call
included a start.time argument, that value is of course
retained.
One problem with this choice is that some functions must choose a
starting point, plots for example.
This utility function is used by plot.survfit and
summary.survfit to do so, adding a new time point at the front of each
curve in a consistent way: the optional argument to the survfit0
function as the first choice (if supplied),
then the user's start.time if present,
otherwise min(0, x$time). The resulting object is not
guarranteed to work with functions that further manipulate a
survfit object such as subscripting, aggregation, pseudovalues,
etc. (remember the 20 errors). Rather it is intended as a penultimate
step, most often when creating a plot.