Which statement about median survival time is true?

• A. The mean time to death for all participants.
• B. The last observed time point in the study.
• C. The time by which 25% of participants have died.
• D. The time by which 50% of the study group have died.

Answer: (D)

In survival analysis, the median survival time is the time at which half of the participants have experienced the event (death). It represents the 50th percentile of the time-to-event distribution, and we often read it from a Kaplan-Meier survival curve as the point where the estimated survival probability crosses 0.5. This measure is more robust than the mean when there are long or censored survival times, because extreme values can heavily influence the average. It is not the mean time to death, nor the last observed time point, nor the time by which 25% have died (that would be the 25th percentile).