These studies usually compare two groups of people (or, cases): Those with a disease or an outcome variable of interest (the focus of the study) and those without the disease (or, controls). This second group is commonly referred to as the control group (or, comparison group or the reference group).

The occurrence of the possible cause is then compared between the cases and the controls. Data from more than one point in time are collected, unlike cross-sectional studies which collect data for only one point in time. Thus, case-control studies are longitudinal studies that GO BACK in time, from the occurrence of the disease to a possible cause.