Which of the following correctly defines right, interval, and left censoring?

• A. Right: when the event is observed exactly at the time it occurs; Interval: when the event is observed at precise times; Left: when the event is observed at the start
• B. Right: when the event occurs before observation begins; Interval: when the event occurs exactly at an observation; Left: the interval occurs at the start
• C. Right: the event has not occurred by the end of observation or the subject is lost to follow-up; Interval: when the event is observed to occur between two observation times; Left: when the event is known to occur before observation begins
• D. Right: when the event time is exactly known; Interval: when the event never observed; Left: observation ends at study conclusion

Answer: (C)

In survival analysis, censoring describes incomplete information about when the event happens. Right censoring means we don’t observe the event by the end of the study or the person is lost to follow-up, so we only know that the event would occur after a certain time. Interval censoring occurs when we know the event happened somewhere between two observation times, but not the exact moment. Left censoring means the event is already present before observation begins, so the time of occurrence is known to be before the study starts.

This set of definitions matches the best option: right censoring is about events not yet occurred by the end or loss to follow-up, interval censoring is when the event is observed to occur between two checks, and left censoring is when the event is known to occur before observation begins. The other descriptions imply exact timing or mislabel the timing relative to observation in ways that don’t reflect how censoring is defined.