Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 207 / 4 / a / i / Solution

The risk set at event time $a_j$ contains individuals still under observation and event-free immediately before $a_j$. For group $k$ its size isThe use of $\ge$ keeps the individual who experiences the event at $a_j$ in the risk set just before that event.