An epidemiologist is evaluating a newly implemented screening program for hypertension in a factory with 10,000 workers. On the first screening day (baseline), 1,200 workers are found to have hypertension. Over the next 12 months, among the 8,800 workers who were initially hypertension-free, 440 develop hypertension. The epidemiologist wants to determine which investigation/measure would be most appropriate to quantify the *new occurrence* of hypertension in this initially disease-free cohort.

Explanation

## Measuring New Disease Occurrence in a Cohort **Key Point:** The question asks for the measure that quantifies the *new occurrence* (incidence) of hypertension in an initially disease-free population over a fixed 12-month period. This is the definition of cumulative incidence. ### Cumulative Incidence Calculation Cumulative incidence (CI) = $\frac{\text{Number of new cases during follow-up}}{\text{Population at risk at baseline}}$ In this scenario: - New cases of hypertension in 12 months = 440 - Population at risk (initially hypertension-free) = 8,800 - Cumulative incidence = 440 / 8,800 = 0.05 or **5%** **Interpretation:** 5% of initially hypertension-free workers developed hypertension over 12 months. This is the *risk* of developing hypertension in this cohort. ### Why Cumulative Incidence Is the Best Choice **High-Yield:** Cumulative incidence is the standard measure for: 1. Assessing risk in a defined cohort 2. Comparing disease occurrence between groups 3. Answering: "What proportion of disease-free people will develop disease?" 4. Fixed follow-up periods with complete or near-complete follow-up ### Comparison Table | Measure | Numerator | Denominator | Time Frame | Answers | |---------|-----------|-------------|-----------|----------| | **Cumulative Incidence** | New cases only | Disease-free at start | Fixed period | "What is the risk?" | | **Baseline Prevalence** | All existing cases | Total population | Point in time | "What is the burden?" | | **Period Prevalence** | All cases (old + new) | Average population | Time interval | "What is the overall burden over time?" | | **Incidence Density** | New cases | Person-time at risk | Variable follow-up | "What is the rate per unit time?" | **Clinical Pearl:** In occupational health and workplace screening programs, cumulative incidence is used to evaluate the effectiveness of interventions and to assess the risk of disease development in the workforce over a defined period. **Mnemonic:** **CI = NEW cases / AT-RISK population** — Cumulative Incidence always measures *new* disease in those initially *disease-free* over a *defined time*.