Add the point for a starting time ("time 0") to a survfit object's elements. This is useful for plotting.
survfit0(x, ...)a reformulated version of the object with an initial data point 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.
a survfit object
any other arguments are ignored
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.
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
extrapolated that it might break 1/3 of the other CRAN packages
that depend on survival, if made a default.)
Many curves do include a value t0 for "time 0",
which is where the survfit
routine has surmised that the curve would start.
One problem with this choice is that some functions must choose a
starting point, plots and computation of the restricted mean survival
time are two primary examples.
This utility function is used by plot.survfit and
summary.survfit to fill in that gap.
The value used for this first time point is the first one below
a t0 value found in the in the object.
for single state survival
min(0, time) for Surv(time, status) data
min(time1) for Surv(time1, time2, status) data
for multi state survival
min(0, time) for Surv(time, event) data, e.g., competing risks
min(time1) for Surv(time1, time2, event) data, if everyone starts in the same state
(Remember that negative times are allowed in Surv objects.)
This function will add a new time point at the front of each curve, but only if said time point is less than existing points in the curve. If there were a death on day 0, for instance, it will not add a (time=0, survival=1) point. (The question of whether the plotted curve in this case should or should not start with a vertical segment can be debated ad nauseum. It has no effect on the area under the curve (RMST), and the summary for time 0 should report the smaller value.)
The resulting object is not currently
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 or summary of the curve(s).