In a case-control study, the measure of association is the?

• A. Prospective follow-up
• B. Measures risk ratio
• C. Odds ratio
• D. Randomizes exposures

Answer: (C)

Case-control studies compare how often exposure occurred among people with the disease (cases) versus those without the disease (controls). Because you design the study by selecting cases and controls, you don’t have information on the actual incidence or risk of disease in exposed and unexposed groups, so you can’t directly calculate a risk ratio. Instead, you look at the odds of having been exposed in the cases compared with the odds of exposure in the controls. This ratio of odds is the odds ratio, which measures the strength of the association between exposure and disease. When the disease is rare, the odds ratio closely approximates the relative risk.