A 42-year-old woman from rural Maharashtra presents with a 6-month history of progressive dyspnea and cough. Chest X-ray shows bilateral upper lobe fibrosis. She reports exposure to silica dust while working in a stone quarry for 15 years. A case–control study is conducted to investigate the association between silica exposure and silicosis in quarry workers. Among 200 exposed workers with silicosis, 180 had documented silica exposure history. Among 200 unexposed controls without silicosis, 30 had documented silica exposure history. Which measure of association is most appropriate to calculate from this case–control study design, and what does it estimate?

Explanation

## Study Design Context This is a **case–control study** — participants are selected based on disease status (silicosis present or absent), and exposure history is determined retrospectively. The study design determines which measure of association can be validly calculated. ## Why Odds Ratio is the Correct Measure **Key Point:** In a case–control study, the **Odds Ratio (OR)** is the only measure of association that can be directly and unbiasedly calculated from the study data. The OR is calculated as: $$OR = \frac{\text{Odds of exposure in cases}}{\text{Odds of exposure in controls}} = \frac{a/b}{c/d}$$ Where: - a = exposed cases = 180 - b = unexposed cases = 20 - c = exposed controls = 30 - d = unexposed controls = 170 $$OR = \frac{180/20}{30/170} = \frac{9}{0.176} \approx 51$$ This OR of ~51 indicates that the odds of silica exposure are approximately 51 times higher in workers with silicosis compared to those without. ## Why Relative Risk Cannot Be Directly Calculated **High-Yield:** Case–control studies do NOT allow direct calculation of **Relative Risk (RR)** because: 1. The sampling fraction (how many exposed vs. unexposed individuals were enrolled) is determined by the investigator, not by the natural disease incidence in the population. 2. RR requires knowledge of incidence or prevalence in exposed and unexposed populations, which case–control studies do not provide. 3. Attempting to calculate RR from case–control data introduces bias. ## When OR Approximates RR **Clinical Pearl:** When the disease is **rare** (prevalence < 10%), the OR approximates the RR. In silicosis (an occupational disease with moderate prevalence in exposed workers), this approximation may hold, but it is not a reason to use OR — OR is used because it is the only valid measure for case–control designs. ## Comparison Table | Measure | Study Design | Can Be Directly Calculated? | Interpretation | | --- | --- | --- | --- | | **Relative Risk (RR)** | Cohort, Cross-sectional | Yes | Ratio of disease risk in exposed vs. unexposed | | **Odds Ratio (OR)** | Case–control, Cohort, Cross-sectional | Yes (all designs) | Ratio of odds of exposure in cases vs. controls | | **Attributable Risk** | Cohort, Cross-sectional | Yes | Absolute risk difference | **Warning:** Do not confuse the ability to calculate OR in case–control studies with the ability to calculate RR. RR requires prospective follow-up data or population-level incidence rates.