## Incidence Definition and Calculation **Key Point:** Incidence is the number of **new cases** of a disease that occur in a population during a specified time period. It measures the risk of developing the disease and is always expressed as a rate (per unit population per unit time). ### Formula $$\text{Incidence} = \frac{\text{Number of new cases during the period}}{\text{Population at risk during the period}} \times k$$ where $k$ is the multiplier (e.g., 100,000 for per 100,000 population). ### Calculation for This Question - **Number of new TB cases during 1 April 2024 to 31 March 2025:** 80 - **Population at risk:** 200,000 - **Incidence** = $\frac{80}{200,000} \times 100,000 = 40$ per 100,000 population per year ### What NOT to Include in Incidence - **Deaths (60 individuals):** Deaths do not affect incidence calculation; incidence counts new cases, not outcomes. - **Prevalence change (400 → 420):** This reflects the net change from incidence minus deaths and recoveries, not incidence alone. - **Previously known cases:** Only new cases are counted in incidence. **High-Yield:** Incidence is a **forward-looking** measure (new cases going forward), while prevalence is a **snapshot** measure (all cases at a point in time). Incidence is always ≤ prevalence because incidence is a subset of prevalent cases. **Mnemonic:** **I**ncidence = **I**ncident (new event), **P**revalence = **P**oint-in-time (snapshot). **Clinical Pearl:** In TB surveillance, incidence is the key epidemiological indicator for assessing disease control and transmission dynamics. It is independent of mortality and recovery rates.
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