What is the measure of association used in a case-case study?

• A. Relative risk
• B. Incidence rate ratio
• C. Hazard ratio
• D. Odds ratio

Answer: (B)

In a case-case study you’re not estimating how often a disease occurs in the general population or comparing time-to-event outcomes. Instead, you’re comparing two groups of cases to see if a exposure differs between them. Because there’s no population denominator or person-time to form risks or rates, the appropriate summary measure is the odds ratio. It reflects how much more (or less) likely exposure is in one case group compared with the other, and can be estimated with simple cross-tabulation or logistic regression.

Relative risk and incidence rate ratio require incidence data from at-risk populations, which isn’t available in a case-case comparison. Hazard ratio comes from time-to-event analysis and isn’t the standard for this design.

Example: if exposure X is present in 60 of 100 cases with disease A and in 30 of 100 cases with disease B, the odds ratio is (60/40) ÷ (30/70) ≈ 3.5, indicating exposure X is more common in disease A cases than in disease B cases.