What is the difference between cohort and case-control studies?

What is the difference between cohort and case-control studies?

• A. A cohort follows exposure status to observe outcomes and yields risk ratios; a case-control starts with cases and controls to assess prior exposure, yielding odds ratios.
• B. A cohort starts with cases and controls; a case-control follows exposure status.
• C. A cohort uses cross-sectional data; a case-control uses longitudinal data.
• D. A cohort estimates incidence; a case-control estimates prevalence.

Answer: (A)

The key difference lies in how participants are selected and what measure of association is produced. In a cohort study you start by grouping people by whether they have been exposed, then follow them over time to see who develops the outcome. This setup lets you observe incidence in each group and directly calculate a risk (incidence) and a relative risk (risk in the exposed vs unexposed). In a case-control study you start with people who already have the outcome (cases) and those without it (controls), then look back to determine their prior exposure. Because you don’t track incidence in this design, you estimate the odds of exposure and report an odds ratio as the measure of association. The odds ratio approximates the relative risk when the outcome is rare, but it is not a direct measure of incidence. Cohort studies are generally stronger for establishing temporality and estimating incidence directly, though they can be more resource-intensive. Case-control studies are efficient for studying rare outcomes and multiple exposures but are more prone to biases such as recall and selection bias.

The key difference lies in how participants are selected and what measure of association is produced. In a cohort study you start by grouping people by whether they have been exposed, then follow them over time to see who develops the outcome. This setup lets you observe incidence in each group and directly calculate a risk (incidence) and a relative risk (risk in the exposed vs unexposed).

In a case-control study you start with people who already have the outcome (cases) and those without it (controls), then look back to determine their prior exposure. Because you don’t track incidence in this design, you estimate the odds of exposure and report an odds ratio as the measure of association. The odds ratio approximates the relative risk when the outcome is rare, but it is not a direct measure of incidence.

Cohort studies are generally stronger for establishing temporality and estimating incidence directly, though they can be more resource-intensive. Case-control studies are efficient for studying rare outcomes and multiple exposures but are more prone to biases such as recall and selection bias.