## Measuring New Disease Occurrence in Cohort Studies **Key Point:** In a prospective cohort study, the most commonly reported measure of new disease occurrence is **cumulative incidence (risk)**, which is the proportion of disease-free individuals who develop the disease during the follow-up period. ### Cumulative Incidence vs. Incidence Rate | Measure | Formula | Denominator | Time Component | Best For | |---------|---------|-------------|-----------------|----------| | **Cumulative Incidence (Risk)** | New cases / Population at risk (disease-free at baseline) | Fixed population | Fixed follow-up period (e.g., 2 years) | Cohort studies with defined follow-up period | | **Incidence Rate** | New cases / Person-time at risk | Person-time (accounts for variable follow-up) | Per unit time (e.g., per 1000 person-years) | Populations with variable follow-up duration | | **Point Prevalence** | Cases at time t / Total population at time t | All individuals (including prevalent cases) | Single point in time | Cross-sectional surveys | **High-Yield:** In this cohort study: - **Cumulative Incidence = 40/2000 = 0.02 or 2%** — the proportion of initially disease-free individuals who developed TB over 2 years - This is the most intuitive and commonly reported measure in cohort studies with uniform follow-up - Incidence rate would require accounting for person-time, which is more complex but useful when follow-up duration varies **Clinical Pearl:** Cumulative incidence is also called **risk** because it represents the probability that a disease-free individual will develop the disease within a specified time period. It ranges from 0 to 1 (or 0% to 100%). **Mnemonic:** **CIR** — **C**umulative **I**ncidence = **R**isk in a fixed cohort over a fixed period.
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