In a classic case-control study, which measure of association is typically used to estimate disease risk?

• A. Odds ratio
• B. Relative risk
• C. Risk difference
• D. Hazard ratio

Answer: (A)

In a case-control study you start with people who have the disease and those who don’t, then look back to see who was exposed. Because the study is organized around disease status, you don’t know how many people in the underlying population were at risk, so you can’t directly compute incidence or risk by exposure. The measure that naturally comes from this design is the odds ratio, which compares how the odds of having been exposed differ between cases and controls. If exposure is more common among cases, the odds ratio exceeds one, indicating an association between exposure and disease. The odds ratio is calculated as (exposed cases / unexposed cases) divided by (exposed controls / unexposed controls). When the disease is rare, this odds ratio approximates the relative risk, which is why it’s used to estimate disease risk in this design. Other measures like relative risk or risk difference require incidence data from a cohort, and a hazard ratio comes from time-to-event analyses, not a classic case-control setup.