How do cohort studies and case-control studies differ?

Cohort studies and case-control studies are both observational study designs used to investigate associations between exposures and outcomes, but they differ fundamentally in their methodology and the types of measures they produce.

Cohort studies track groups of individuals who are exposed to a certain risk factor and compare them to a group that is not exposed. This design allows researchers to calculate incidence rates of disease (the number of new cases that develop over a period) and thus measure the risk of an outcome occurring in the exposed group versus the non-exposed group. The most common measure calculated in cohort studies is the risk ratio (also called relative risk), which provides a direct measure of the strength of the association between exposure and outcome.

The correct choice emphasizes that cohort studies typically measure risk ratios. This is because they follow groups over time and can directly observe how many individuals develop the disease in each group, allowing for a direct comparison of risk between those exposed and those not exposed.

In contrast, case-control studies start with individuals who have the outcome (cases) and compare them to individuals without the outcome (controls). These studies work backward from the outcome to assess past exposures, meaning they calculate odds ratios instead of risk ratios. They cannot measure incidence directly, as they do not follow participants