## Relative Risk Calculation **Key Point:** Relative Risk (RR) in a cohort study is calculated as: $$RR = \frac{\text{Risk in exposed group}}{\text{Risk in unexposed group}} = \frac{\text{Incidence in exposed}}{\text{Incidence in unexposed}}$$ ### Step-by-Step Calculation 1. **Risk in smokers (exposed)** = 100 / 1000 = 0.10 (or 10%) 2. **Risk in non-smokers (unexposed)** = 10 / 1000 = 0.01 (or 1%) 3. **Relative Risk** = 0.10 / 0.01 = **10.0** ### Interpretation **High-Yield:** An RR of 10.0 means that smokers are **10 times more likely** to develop lung cancer compared to non-smokers over the 10-year follow-up period. ### When to Use Relative Risk - **Cohort studies** (prospective or retrospective) - **Randomized controlled trials** (RCTs) - When you can **directly measure incidence** in both exposed and unexposed groups - When disease is **common** (>10%) **Clinical Pearl:** RR is the most intuitive measure of association because it directly compares the probability of disease in two groups — it answers "How much more likely is disease in the exposed group?" ### Key Distinction from Odds Ratio | Measure | Design | Calculation | Use Case | |---------|--------|-------------|----------| | **Relative Risk** | Cohort, RCT | Incidence(exposed) / Incidence(unexposed) | Direct comparison of disease risk | | **Odds Ratio** | Case-control | Odds(exposure\|disease) / Odds(exposure\|no disease) | Backward reasoning from disease to exposure | **Mnemonic:** **COHORT = RR** (Cohort studies use Relative Risk); **CASE-CONTROL = OR** (Case-Control studies use Odds Ratio). **Warning:** When disease is rare, OR ≈ RR, but when disease is common, OR is substantially larger than RR.
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