What is left truncation (delayed entry) in survival analysis, and when is it used in cohort studies?

• A. Individuals enter the study after exposure or disease onset; used to adjust for late entry
• B. Excluding early events from analysis
• C. Random assignment of entry times
• D. Eliminating censored observations

Answer: (A)

Left truncation, or delayed entry, means you start counting a participant’s time at the moment they actually enter the study, which is after their exposure or disease onset. In survival analysis, this ensures the risk set at any time only includes people who have already entered by that time and have not yet had the event. It’s used in cohort studies precisely when enrollment happens later than the start of the risk period, so you don’t bias results by ignoring the fact that some individuals had to survive up to their entry time to be observed. If you didn’t account for this, those who die or experience the event before entry would be missing, artificially inflating survival estimates because only longer-surviving individuals enter the study.

So the correct statement captures that people enter after exposure or disease onset and that left truncation adjusts for late entry. Excluding early events, randomizing entry times, or eliminating censored observations do not describe left truncation.