In a cohort study examining the association between smoking and lung cancer, the relative risk (RR) is calculated as 4.5. What does this RR value indicate?

Explanation

## Understanding Relative Risk in Cohort Studies **Key Point:** Relative Risk (RR) is the ratio of the incidence of disease in the exposed group to the incidence in the unexposed group. It is the primary measure of association in cohort studies. ### Interpretation of RR = 4.5 An RR of 4.5 means: - The risk (probability) of developing lung cancer in smokers is 4.5 times the risk in non-smokers - If baseline risk in non-smokers is 10%, the risk in smokers would be 45% - This is a **prospective measure** calculated from cohort data ### Formula $$RR = \frac{\text{Incidence in exposed}}{\text{Incidence in unexposed}} = \frac{I_e}{I_u}$$ **High-Yield:** RR > 1 indicates increased risk; RR < 1 indicates protective effect; RR = 1 indicates no association. ### Why This Differs from Other Measures | Measure | Study Type | Calculation | Interpretation | |---------|-----------|-------------|----------------| | **Relative Risk (RR)** | Cohort | Incidence ratio | Times more/less likely to develop disease | | **Odds Ratio (OR)** | Case-Control | Odds ratio | Odds of exposure in cases vs controls | | **Attributable Risk** | Cohort | Risk difference | Absolute excess cases per population | | **Incidence Rate** | Cohort | Cases per person-years | Absolute frequency of new cases | **Clinical Pearl:** An RR of 4.5 is a strong association and would typically be considered statistically and clinically significant if the 95% CI does not cross 1.0.