Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 207 / 5 / vii / Solution

Every patient must either have an observed death or at least 48 months of follow-up. The unresolved patient is D, who starts in month 12 and completes 48 months at the end of month 59. Therefore the month-60 analysis is the earliest twelve-monthly analysis at which ${\hat{F}}_{\infty }(48)$ is evaluable.