The <risk set> at $x_i$ contains every individual still under observation and event-free immediately before $x_i$. Since the observed times are strictly ordered increasingly, these are individuals $i,i+1,\ldots,n$, so its size is
$$
Y(x_i)=n-i+1.
$$

Solution

Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 207 / 4 / a / i / Solution

 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/iii/paper-207/4/a/i/solution