In an open (dynamic) cohort, how are entries of new participants typically treated in analysis?

• A. They do not contribute any person-time.
• B. Entry of new participants is ignored in analysis.
• C. New participants contribute to person-time from their entry date, with censoring at loss to follow-up or end of study.
• D. Entries require randomization.

Answer: (C)

In an open (dynamic) cohort, participants can join at different times, and their risk starts from the moment they enter the study. In analysis, each new participant adds to the total person-time from their entry date, and their follow-up is censored at loss to follow-up or at the end of the study. This approach lets you accurately estimate incidence rates and perform time-to-event analyses that account for varying entry times. They are not ignored, and randomization is not required for entry.