In a case-control study on lung cancer, 70% of cases and 40% of controls smoked. The Odds Ratio (OR) is calculated. Compared to a cohort study with the same population, the OR will approximate RR only when:

• A The disease is common (prevalence >10%)
• B A nested case-control design is used
• C The exposure prevalence among controls exceeds 20%
• D The disease is rare (<1%) and controls are sampled from the source population ✓

Explanation

The rare disease assumption states that OR approximates RR only when disease prevalence is very low (<1%), because under this condition the odds of disease in exposed and unexposed approximate their respective risks. When controls represent the source population (density sampling), the OR directly estimates the incidence rate ratio. For common diseases, OR overestimates RR when RR>1 and underestimates it when RR<1.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.