An epidemiologist in Mumbai is investigating a cluster of oral cancers among betel quid chewers. She identifies 120 cases of oral cancer diagnosed in the past 2 years and recruits 240 age-matched controls without oral cancer from the same neighborhoods. Both groups are interviewed about their betel quid chewing habits, frequency, and duration. The study aims to calculate the association between betel quid chewing and oral cancer risk. Which statement best describes why this study design was chosen over a cohort approach?

Explanation

## Why Case-Control Design Is Optimal for Rare Diseases ### Efficiency in Rare Disease Investigation **Key Point:** Oral cancer is a **rare disease** (incidence ~5–10 per 100,000 in India). A case-control design is far more efficient than a prospective cohort study because: 1. **Cohort approach would be impractical:** You would need to enroll tens of thousands of betel quid chewers and follow them for 5–10 years to accumulate 120 cases — prohibitively expensive and time-consuming. 2. **Case-control approach is efficient:** Identify 120 existing cases and 240 controls, interview both groups about past exposure — completed in months, not years. ### Comparison: Cohort vs Case-Control for Rare Diseases | Aspect | Cohort Study (Rare Disease) | Case-Control Study (Rare Disease) | |--------|------------------------------|-----------------------------------| | **Sample size needed** | 50,000–100,000+ | 120 cases + 240 controls | | **Follow-up time** | 5–10 years | None (retrospective) | | **Cost** | Very high | Moderate | | **Time to completion** | Years | Months | | **Feasibility** | Poor | Excellent | | **Measure of association** | RR (direct) | OR (approximates RR when disease is rare) | **High-Yield:** When disease is rare: - **Use case-control** (efficient, practical) - When exposure is rare: - **Use cohort** (efficient to find exposed individuals) ### Why Case-Control Odds Ratio Works for Rare Diseases **Clinical Pearl:** In rare diseases, the **odds ratio (OR) from a case-control study approximates the relative risk (RR)** from a cohort study: $$OR \approx RR \text{ when disease incidence} < 10\%$$ Oral cancer incidence is <1%, so OR is an excellent proxy for RR. **Mnemonic: RARE for Case-Control Suitability** - **R**are disease (oral cancer) - **A**ssociation measured by OR - **R**etrospective (efficient) - **E**xposure recalled from cases and controls [cite:Park 26e Ch 8]