## Measures of Association in Cohort Studies **Key Point:** Cohort studies follow exposed and unexposed groups forward in time to measure disease incidence, allowing direct calculation of relative risk (RR). ### Why Relative Risk is the Primary Measure In a cohort study: 1. Participants are classified by **exposure status** at baseline 2. Both groups are followed prospectively 3. **Incidence** of disease is measured in each group 4. RR = Incidence in exposed / Incidence in unexposed Since cohort studies measure incidence directly (not odds), relative risk can be calculated without assumptions. ### Comparison of Measures by Study Design | Measure | Cohort | Case-Control | Cross-sectional | |---------|--------|--------------|------------------| | **Relative Risk** | Directly calculated | Estimated indirectly | Can be calculated | | **Odds Ratio** | Can be calculated | Directly calculated | Can be calculated | | **Incidence** | Measured directly | Cannot be measured | Cannot be measured | | **Prevalence** | Can be measured | Cannot be measured | Measured directly | **High-Yield:** The fundamental advantage of cohort studies is the ability to measure **incidence** and thus calculate **relative risk** directly — this is what distinguishes them from case-control studies, which must use odds ratio as a proxy. **Clinical Pearl:** When RR = 1, there is no association; RR > 1 indicates increased risk; RR < 1 indicates protective effect.
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