In a cohort study comparing smokers and non-smokers for lung cancer risk, the relative risk was 12. In a subsequent case-control study on the same population, the odds ratio was 14.4. This difference is BEST explained by the fact that:

• A The case-control study overestimated because the disease is rare
• B The odds ratio approximates the relative risk only when disease prevalence is low (<10%)
• C The odds ratio overestimates the relative risk when disease prevalence is high (>10%) ✓
• D Cohort studies always underestimate risk compared with case-control designs

Explanation

The odds ratio (OR) is a valid approximation of relative risk (RR) only when the outcome is rare (prevalence <10%) — this is the 'rare disease assumption.' When disease prevalence is high, OR diverges upward from RR: OR = RR × [(1 − P0)/(1 − P1)], where P1 > P0, making OR > RR. Since lung cancer prevalence in the cohort is high enough to inflate OR to 14.4 from RR 12, option C is correct. Cohort studies do not systematically underestimate risk.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.