## Direct Calculation in Cohort Studies **Key Point:** Cohort studies follow exposed and unexposed groups forward in time and directly measure the incidence of disease in each group. This allows direct calculation of **relative risk (RR)**. ### Relative Risk Calculation Relative risk is computed as: $$RR = \frac{\text{Incidence in exposed}}{\text{Incidence in unexposed}} = \frac{a/(a+b)}{c/(c+d)}$$ Where: - a = exposed with disease - b = exposed without disease - c = unexposed with disease - d = unexposed without disease ### Why RR is Direct in Cohort Studies Because cohort studies: 1. Start with **exposure status known** 2. Follow subjects **forward in time** 3. Measure **incidence** (new cases) in each exposure group 4. Can directly compare disease occurrence rates **High-Yield:** Odds ratio is the measure calculated directly in **case-control** studies (which start with disease status). Attributable fractions and population attributable risk are **derived measures** calculated from RR, not directly measured. **Clinical Pearl:** RR > 1 indicates increased risk; RR < 1 indicates protective effect; RR = 1 indicates no association. [cite:Park 26e Ch 9]
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