Case-control studies are observational studies where the outcome (disease/illness) is measured before the exposure is observed. In other words, we know what the outcome is, but we do not know what caused it (the exposure).
Unlike cohort studies which are suitable for studying rare exposures, case-control studies are useful for studying rare outcomes. In fact, the hardest part of a case-control study is ensuring that those in your control group (your controls) are similar in every aspect to your cases except on the outcome. Your controls should not have the disease that led your cases to become cases.
In case-control studies, both the exposure and the outcome have already occurred. The job of the researcher is to determine what/which exposure led to the outcome. Again, this is different from the cohort study where the researcher follows a group of people across time to learn if those who were exposed to a particular event (e.g., hazardous chemical spill) developed an outcome (an illness or disease).
Though usually less expensive than cohort studies, case-control studies are prone to selection and recall bias. With case-control studies, you can usually only predict an estimate of risk.
Also published on Medium.