Case control studies: Case-control studies are those in which persons with a specified condition (the cases) and pesons without the condition (the controls) are selected for study. The proportion of cases and controls with certain characteristics or exposure is then measured and compared. For example, knowing that there are 10 school children with purple spots in grade 3, a set of other third grade children from the same school but without purple spots would be identified as controls, and analysis done to see what different exposures the purple-spotted children had than the non-spotted.

Cohort studies: groups of individuals with some common feature (age and geography, for example) are identified for study over time to learn about differing health and illness experiences. For example, one might enroll in a study all third graders in a school and follow them until graduation, attempting to identify the differences in experiences of those who maintained a body weight close to recommended and those who did not.