 Risk set

The risk set immediately before an event time contains the individuals who remain observed, event-free, and eligible to experience the event. For a cohort observed from time zero, with exit times $Y_i=\min (T_i,C_i)$ under right censoring, write $R_j=\{i:Y_i\ge t_j\}$ and $n_j=|R_j|$. Individuals with an event or right censoring exactly at $t_j$ are included in this denominator; process events before removing subjects right-censored at that same recorded time. Earlier right censoring removes subjects from later risk sets. With delayed entry, use the entry times too, as in the Kaplan–Meier estimator with delayed entry.