A public health researcher in Delhi is analyzing data from a cross-sectional survey of 5000 adults to assess the association between smoking and chronic obstructive pulmonary disease (COPD). The study found that among 1000 smokers, 200 had COPD, and among 4000 non-smokers, 100 had COPD. The researcher has calculated both the relative risk (RR = 4.0) and the odds ratio (OR = 2.67) for this association. However, the researcher is uncertain about which measure to report and how to interpret the findings in the context of public health policy recommendations. What is the most appropriate next step in analyzing and presenting these epidemiological findings?

Explanation

## Study Design and Choice of Measure **Key Point:** In cross-sectional studies, relative risk (RR) is the most appropriate and interpretable measure of association when the outcome (disease) prevalence is not rare (typically >10%). ### Why RR is Preferred Here **High-Yield:** The prevalence of COPD in this study is: - Among smokers: 200/1000 = 20% - Among non-smokers: 100/4000 = 2.5% - Overall: 300/5000 = 6% Since the disease prevalence exceeds 10% in the exposed group, RR is the most direct and interpretable measure. ### RR vs OR in Different Study Designs | Study Design | Preferred Measure | Reason | |---|---|---| | **Cross-sectional** (disease not rare) | Relative Risk | Direct calculation; intuitive interpretation | | **Cross-sectional** (disease rare, <10%) | Odds Ratio | RR and OR converge; OR acceptable | | **Case-control** | Odds Ratio | RR cannot be calculated; OR approximates RR when disease is rare | | **Cohort** | Relative Risk | Direct calculation of incidence rates | **Clinical Pearl:** RR of 4.0 means smokers are 4 times more likely to have COPD compared to non-smokers — this is the most intuitive interpretation for public health messaging. ### Interpretation of Findings **Mnemonic: RARE** — **R**elative Risk when disease is **A**vailable (calculable), **R**are or not, **E**xposed group prevalence determines choice. - RR = 4.0: Smokers have 4-fold increased odds of COPD - OR = 2.67: The odds of being a smoker among those with COPD vs without COPD is 2.67:1 - RR is more clinically meaningful for policy (e.g., "smoking increases COPD risk 4-fold") ### Public Health Application **Key Point:** When communicating findings to policymakers and the public, RR is more intuitive and actionable than OR. An RR of 4.0 is a clear, powerful message for tobacco control initiatives. [cite:Park 26e Ch 10]