Which method calculates survival probability at each event time without grouping into intervals?

• A. Life-table method
• B. Relative survival
• C. Kaplan-Meier method
• D. Median survival time

Answer: (C)

Kaplan-Meier estimation calculates survival probabilities at each observed event time, producing a stepwise survival curve that updates only when an event occurs. At each event time, the survival probability is updated using S(t) = S(t−) × (1 − d_i/n_i), where d_i is the number of events (deaths) at that time and n_i is the number at risk just before that time. Censored observations are handled by reducing the number at risk for future time points but do not cause a drop in the survival estimate, so the curve steps down only when an event happens. This exact-event-time approach contrasts with the life-table method, which groups times into predefined intervals and estimates survival within those intervals, introducing interval-based assumptions. Median survival time is just a single summary statistic (the time when survival drops to 50%), and relative survival compares observed survival to an expected survival in a reference population rather than calculating pointwise survival probabilities.