What’s the difference between a case-control study and a cohort study?

A case-control study differs from a cohort study because cohort studies are more longitudinal in nature and do not necessarily require a control group.

While one may be added if the investigator so chooses, members of the cohort are primarily selected because of a shared characteristic among them. In particular, retrospective cohort studies are designed to follow a group of people with a common exposure or risk factor over time and observe their outcomes.

Case-control studies, in contrast, require both a case group and a control group, as suggested by their name, and usually are used to identify risk factors for a disease by comparing cases and controls.