In a cohort study, which of the following is the PRIMARY measure of association used to quantify the relationship between exposure and outcome?

Explanation

## Primary Measure of Association in Cohort Studies **Key Point:** In cohort studies, the **relative risk (RR)** is the primary measure of association because cohort studies follow exposed and unexposed groups forward in time to observe the development of disease. ### Why Relative Risk? Cohort studies are **prospective** (or can be retrospective but still follow the cohort structure). Because the investigator: 1. Identifies exposed and unexposed individuals at baseline 2. Follows them forward to measure disease incidence in each group 3. Can directly calculate the **incidence in exposed** and **incidence in unexposed** The relative risk is calculated as: $$RR = \frac{\text{Incidence in exposed}}{\text{Incidence in unexposed}} = \frac{a/(a+b)}{c/(c+d)}$$ where the 2×2 table has disease (yes/no) as columns and exposure (yes/no) as rows. ### Comparison with Case-Control Studies | Feature | Cohort Study | Case-Control Study | |---------|--------------|--------------------| | **Primary measure** | Relative Risk (RR) | Odds Ratio (OR) | | **Direction** | Exposure → Disease | Disease ← Exposure | | **Incidence calculable?** | Yes | No | | **Why OR in case-control?** | Cannot calculate true incidence; disease prevalence in sample is artificially fixed by design | — | **High-Yield:** RR directly answers "How many times more likely is disease in the exposed group?" This is only possible in cohort studies where you follow groups forward and measure true incidence. **Clinical Pearl:** An RR of 2.5 means exposed individuals are 2.5 times more likely to develop the disease than unexposed individuals—a direct, intuitive interpretation.