Which statement correctly describes truncation definitions in survival data?

• A. Right truncation is equivalent to right censoring; interval truncation occurs throughout; left truncation at the start
• B. Right truncation means the observation window starts after the event
• C. Interval truncation means the event is always observed exactly
• D. Left truncation has no impact on the data

Answer: (A)

In survival data, truncation means we only observe individuals whose event times fall inside the study’s observation window, so who enters the study and when matters for what data are available. The key idea is how entry time and window endpoints shape who is included.

Left truncation occurs when subjects enter the study after time zero, so we only follow those whose event time is after their entry time. In other words, the data are truncated on the left because early times are missing due to delayed entry. This is the aspect captured by the statement that truncation is at the start.

Right truncation would mean we miss individuals whose event time would occur after the study window ends, while interval (or interval censoring) refers to knowing only that the event lies within some interval rather than its exact time. However, those descriptions are less precise and do not reflect the common, correct interpretation as clearly as the left-truncation-at-entry idea does.

So the part of the statement that correctly describes left truncation occurring at the start aligns with how truncation works in survival data, making that aspect the most accurate piece among the options.