Define truncation in survival data.

• A. Right: periods of time in which nothing is known about the animal in terms of whether or not the event occurred (periods of time are gaps); Left: at the start
• B. Right: when the animal is not observed when the event occurs (most common - lost to follow-up); Interval: when animal is observed periodically; Left: interval occurs at start
• C. Right: when something is missing at random; Interval: when data are missing due to dropout; Left: when data are missing for all
• D. Right: when the event occurs exactly at the end; Interval: when the event is never observed; Left: when the time origin is unknown

Answer: (A)

Truncation in survival data refers to selecting only observations whose event times lie inside a defined observation window, so event times outside that window are not observed at all. This creates two directional effects: left truncation happens because the study only starts observing after some origin, so the time before entry is effectively missing from the data; right truncation occurs when the observation window ends before the event could be observed, leaving periods where we do not know whether the event has occurred. The option describes right-truncation as periods where nothing is known about the event status (gaps) and left-truncation as occurring at the start, which aligns with the idea that truncation arises from the timing of observation relative to the event. In practice, left truncation means delayed entry into the study, and right truncation means the study window ends before some events can be observed.