= Solution
\b[The set must contain every observed event time.] Otherwise a discrete <survival likelihood> would assign probability zero to at least one exact event observation.
The set can include additional times without observed events. Include time zero when events at zero are present, and allow remaining tail probability beyond the fitted range. Estimation of a conditional event mass requires a nonempty <risk set>; once follow-up ends, later probability masses are not identified by the data. The usual <Kaplan–Meier estimator> retains the final estimated <survival probability> as its tail convention.
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