A case-control study is one where researchers identify participants by their outcome status (e.g.: do they have a specific condition or not) and group them accordingly. More frequently, the participants then will be asked about their characteristics and exposures. For example, a case-control might study whether individuals with cancer (outcome) are more or less likely to also report past exposure to pollution than people without cancer.

Case-control studies determine associations, but they do not indicate that there is causality between outcome and exposure. These study designs may sound similar to cohort studies, however, the primary difference lies in how the participants are selected. Where a cohort study identifies participants by their exposure status (exposed vs. not-exposed), case-controls identify participants by their outcome status (health state or disease).