In a cohort study investigating the association between smoking and lung cancer, which measure of association is directly calculated from the study data?

Explanation

## Direct Calculation in Cohort Studies **Key Point:** Cohort studies follow disease-free individuals forward in time and directly measure the incidence of disease in exposed versus unexposed groups, allowing calculation of **relative risk (RR)** as the primary measure of association. ### Why Relative Risk is the Answer In a cohort study: 1. Participants are classified by **exposure status** at baseline 2. Both groups are followed prospectively 3. **Incidence rates** are calculated for each group: - Incidence in exposed = a / (a + b) - Incidence in unexposed = c / (c + d) 4. **Relative Risk = Incidence in exposed / Incidence in unexposed** This is a **direct calculation** because the denominator (total at risk) is known from the study design. ### Comparison with Case-Control Studies | Feature | Cohort Study | Case-Control Study | |---------|--------------|--------------------| | **Primary measure** | Relative Risk (RR) | Odds Ratio (OR) | | **Calculation method** | Direct from incidence | Indirect from odds | | **Denominator known?** | Yes (at-risk population) | No (population source unknown) | | **Time direction** | Forward (prospective) | Backward (retrospective) | **High-Yield:** RR is **only directly calculable in cohort studies** because you know the total number at risk in each exposure group. In case-control studies, you cannot calculate RR directly because you start with cases and controls, not the source population. **Clinical Pearl:** RR > 1 indicates increased risk, RR = 1 indicates no association, RR < 1 indicates protective effect. [cite:Park 26e Ch 10]