Right censoring occurs when

• A. The event is known to occur before observation begins
• B. The event is observed exactly at the time of occurrence
• C. The event has not occurred by the end of observation or the subject is lost to follow-up
• D. The event is observed between two observation times

Answer: (C)

Right censoring happens when the event of interest hasn’t occurred by the time the observation ends or the participant is lost to follow-up, so the exact time of the event is unknown. In this situation you only know that the event time is longer than the last observed time. For example, if you’re tracking time to relapse and a patient is still relapse-free at the study’s end, you know their relapse time exceeds their last visit, but you don’t know when it will actually occur. The same idea applies if a participant drops out—their true event time is beyond their last contact.

This differs from left censoring, where the event occurred before observation began; from having an exact event time with no censoring; or from interval censoring, where you only know the event happened between two observation times.