This type of retrospective study design identifies a group of individuals who have already experienced a particular outcome or who already have a particular condition and compares them to a similar group without the outcome/condition to look for differences between the groups that might reveal associations with the outcome/condition.
Deeper dive
Those who have the condition/outcome of interest are called the “cases.” The “controls” are other individuals who are substantially similar to the cases in important ways, usually matched to the cases on the basis of age, sex, and similar demographic factors, such as geography, race/ethnicity, socioeconomic status, doctor or clinic, etc. How controls are matched to the cases will depend on the study, and many case control studies will match multiple controls to a single case (three controls to one case or 10 to one, etc.).
Researchers then compare certain pre-identified factors or characteristics between the cases and the controls with the hope of identifying risk factors in the case group for the shared condition they have. For example, a case control study might bring together a group of individuals who all have high blood pressure and then match controls without high blood pressure to these cases. Then the researchers might look at dietary patterns or physical activity in the cases and the controls to see if any patterns suggest that certain aspects of diet or physical activity may contribute to high blood pressure.