A case-control study is more efficient than a cohort study when the disease is rare.

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BlackTom AI · Accepted Answer

Consider the question about study design efficiency in the context of a rare disease.
First, recall that a case-control study selects individuals based on disease status (cases with the disease and controls without), which allows researchers to study rare outcomes without having to follow a large cohort over time.
Second, because the disease is rare, identifying enough incident cases in a cohort would require enrolling a very large number of people and following them longitudinally to observe enough cases, which is time-consuming and costly.
Third, in a case-control design, data on past exposures can be collected retrospectively, making it more efficient to evaluate associations between exposures and the rare disease, since the investigator can enrich the sample with cases.
Fourth, the efficiency advantage arises because the study is not constrained by the low incidence of the disease in the general population; instead, the researcher ensures sufficient cases to estimate exposure odds.
Fifth, an important caveat is that case-control studies estimate odds ratios, which approximate relative risk mainly when the disease is rare; they are more susceptible to certain biases (e.g., recall bias) but these do not negate the overall efficiency advantage in rare-disease research.
Sixth, contrasting with a cohort study, a cohort would require large sample sizes and lengthy follow-up to observe enough cases, increasing cost, time, and resource use when the disease is uncommon.
Seventh, while case-control studies are advantageous for rare diseases, they are less suitable for studying multiple outcomes from a single exposure or for estimating absolute risk directly, where cohort designs excel.
Eighth, in summary, for a rare disease, the case-control approach is typically more efficient due to easier case recruitment, retrospective exposure assessment, and reduced need for large, long-term follow-up compared to a cohort study.

PUBHEPI 2410 AU2025 (1751) Quiz Module 7: Case-control studies

A case-control study is more efficient than a cohort study when the disease is rare.

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