## Direct Calculation in Cohort Studies **Key Point:** In cohort studies, the researcher follows exposed and unexposed groups forward in time and directly observes the development of disease in each group. This allows direct calculation of **relative risk (RR)**. ### Why Relative Risk is Directly Calculated Relative risk is computed as: $$RR = \frac{\text{Risk in exposed}}{\text{Risk in unexposed}} = \frac{a/(a+b)}{c/(c+d)}$$ where: - a = disease in exposed - b = no disease in exposed - c = disease in unexposed - d = no disease in unexposed Because cohort studies begin with exposure status and follow participants forward to disease occurrence, both the numerator and denominator (incidence rates) are directly observable. ### Comparison with Case-Control Studies | Measure | Cohort Study | Case-Control Study | |---------|--------------|-------------------| | **Relative Risk** | Directly calculated | Cannot be calculated | | **Odds Ratio** | Can be calculated | Directly calculated | | **Attributable Risk** | Directly calculated | Cannot be calculated | **High-Yield:** Odds ratio is the measure of association in case-control studies because the researcher starts with disease status (cases vs. controls) and looks backward to exposure history. The odds of exposure in cases vs. controls is what is directly observable. **Clinical Pearl:** Population attributable risk and attributable fraction in the exposed are derived measures that require additional information (like prevalence of exposure in the population) and are not directly calculated from the basic 2×2 table alone. ### Mnemonic **"Cohort = RR, Case-Control = OR"** — Cohort studies follow people forward from exposure to disease (Risk), while case-control studies identify odds of past exposure from current disease status (Odds Ratio).
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