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

Let $u_j^{(1)}=u_j$ and $u_j^{(0)}=1-u_j$. Applying the Nelson–Aalen estimator separately to group $k$ giveswith a zero contribution when the event at $a_{\ell }$ occurs in the other group.