What is censoring in survival analysis, and what is right-censoring?

• A. Censoring occurs when the outcome is not observed for a participant during the study; right-censoring means the event has not occurred by study end or loss to follow-up.
• B. Censoring is when data are missing; right-censoring means left-censoring.
• C. Censoring is removing participants who do not complete; right-censoring means excluding those with undetermined outcomes.
• D. Censoring occurs when the outcome is observed; right-censoring means the event has occurred during the study.

Answer: (A)

In survival analysis we track how long until an event occurs, such as death or failure. Censoring happens when we don’t know the exact time of that event for some participants because they either haven’t had the event by the end of the study or they’re lost to follow-up. We still know how long they were observed and that they were event-free up to that point, but the true event time remains unknown beyond that observation time.

Right-censoring is the common form: the participant’s follow-up ends before the event happens, either because the study ends or they are lost to follow-up. We know they survived (or remained event-free) at least up to the last contact, but the exact time to the event is greater than that last observed time.

This aligns with censoring being about incomplete information on the event time rather than simply missing data, and it isn’t about removing participants from the analysis. It also isn’t about the event being observed during the study.