= 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 $R_j=\{i:Y_i\geq t_j\}$ 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>.
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