What is the primary purpose of a case-control study, and what measure of association does it yield?

• A. To study rare outcomes by comparing exposure history; yields relative risk.
• B. To study common outcomes by following exposed and unexposed cohorts; yields risk difference.
• C. To study rare outcomes by comparing exposure history; yields odds ratio.
• D. To measure disease incidence directly in a population; yields incidence rate.

Answer: (C)

Case-control studies are designed to study rare outcomes by identifying people with the outcome (cases) and those without (controls) and then looking back to see their exposure histories. Because you start with the outcome, you can’t directly measure how often the disease occurs in exposed versus unexposed groups in the population, so you don’t estimate incidence or risk. Instead, you compare the odds of exposure among cases to the odds of exposure among controls, yielding an odds ratio as the measure of association. When the outcome is rare, the odds ratio closely approximates the relative risk, which is why this design is favored for studying rare conditions.