A potential drawback of case-cohort design is that it is not optimal if which condition holds?

• A. Exposure changes during the study period
• B. Disease is very common
• C. Covariate data are cheap
• D. The outcome is continuous

Answer: (B)

Case-cohort designs save effort by measuring exposures in a random subcohort and in all cases, which is most efficient when the outcome is rare. When the disease is very common, the number of cases becomes large, so you end up collecting exposure data on a large portion of the cohort anyway. That diminishes the sampling advantage, making the design less efficient and requiring more complex weighting to get correct estimates. In that scenario, a full-cohort approach or another design may be more practical.