Open (dynamic) cohorts are characterized by which of the following?

• A. Fixed cohorts have a defined start and finish with no new members.
• B. Open cohorts continuously enroll new members and follow them over time.
• C. Open cohorts require randomization at enrollment.
• D. Fixed cohorts never lose participants.

Answer: (B)

Open (dynamic) cohorts are defined by ongoing enrollment of new participants and following them over time, rather than fixing the group at a single starting point. This setup reflects real-world populations where people can enter the study at different times and may also leave or be lost to follow-up. Because participants join at various times, analyses rely on person-time to accurately measure rates as the population changes. This is in contrast to fixed cohorts, where the membership is established at the start and no new members are added, though losses to follow-up can occur. Randomization at enrollment is not a defining feature of open cohorts and is more related to experimental designs than to how cohorts are formed. So, continuous enrollment and longitudinal follow-up capture what characterizes an open cohort.