What is the basis of sampling in cohort versus case-control studies?

• A. Cohort studies are based on disease outcome, case-control on exposure.
• B. Cohort studies are based on exposure; case-control on disease outcome.
• C. Both use random sampling from the general population.
• D. Both select by a predefined sampling frame regardless of exposure or outcome.

Answer: (B)

In cohort studies you start by grouping people according to whether they have a particular exposure, then you follow them over time to see who develops the disease. In case-control studies you start with people who already have the disease (cases) and compare them to people without the disease (controls) to look back and assess past exposure.

This basis of sampling is why a cohort design can measure incidence and relative risk directly, since you’re observing outcomes after assigning exposure groups. A case-control design is efficient for studying rare diseases, because you start with cases and controls and look back at exposure history to estimate the association, usually as an odds ratio.

If you hear that a study starts with disease outcomes or that both designs sample randomly from the general population regardless of exposure or outcome, those descriptions don’t match how these designs are actually organized.