## Definition and Distinction **Key Point:** Incidence and prevalence are fundamentally different measures of disease frequency, each serving distinct epidemiological purposes. ### Incidence - Measures **new cases** occurring in a disease-free population over a **defined time period** - Expressed as a rate (cases per person-years at risk) - Reflects the **risk** of developing disease - Useful for understanding disease **etiology** and evaluating **intervention effectiveness** ### Prevalence - Measures **all cases** (both new and existing) **at a single point in time** (point prevalence) or during a **time period** (period prevalence) - Expressed as a proportion (0–1 or percentage) - Reflects the **burden** of disease in a population - Useful for **healthcare planning** and **resource allocation** ### Mathematical Relationship $$\text{Prevalence} \approx \text{Incidence} \times \text{Average Duration of Disease}$$ This relationship shows that prevalence is influenced by both incidence and disease duration; they are **not equivalent**. **High-Yield:** In a stable population with constant incidence and duration, prevalence will increase if either incidence rises or disease duration lengthens (e.g., improved survival in cancer). **Mnemonic:** **I-NEW, P-ALL** — Incidence captures NEW cases; Prevalence captures ALL cases.
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