Distinguish open (dynamic) and closed (fixed) cohorts and discuss implications for analysis.

• A. Open cohort has fixed membership and no new entrants; closed cohort does not.
• B. Open cohort allows new entrants during follow-up and losses; they affect person-time calculation and exposure classification.
• C. Closed cohort permits new entrants during follow-up but forbids losses.
• D. Open cohorts do not permit exposure changes over time.

Answer: (B)

The main idea being tested is how cohort membership changes over time and how that affects analysis. In an open (dynamic) cohort, people can enter the study at different times during follow-up and can also leave or be lost to follow-up. Because of this, the amount of time each person contributes to the study—person-time—varies and needs to be tracked from each individual's entry to their exit. Exposure status and other covariates may also change over time, so analyses must account for time-varying information, often using methods that handle time-dependent covariates (like time-dependent Cox models or Poisson models with person-time data).

This is why the statement describing an open cohort as allowing new entrants during follow-up and losses, and noting that these dynamics affect both person-time calculation and exposure classification, is the best fit. It directly captures the essential implications for analysis.

The other descriptions don’t fit because they describe fixed membership, or they misstate which features belong to open versus closed cohorts, or they imply exposure cannot change over time. In a fixed (closed) cohort, membership is set at baseline with no new entrants, and losses occur only through censoring or death, making person-time and exposure often simpler to handle. In an open cohort, exposure and membership are dynamic, which is the key difference and driving factor for the appropriate analytic approach.