Part 2 · Epidemiology I — Measures and Observational Designs

12. Incidence, Prevalence and the Stock–Flow Relationship

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Same concept, other depth: Foundation →

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Measure	Numerator	Denominator	Interpretation
Incidence proportion (cumulative incidence, risk)	New cases in interval	Disease‑free population at risk at start	Probability over a stated period
Incidence rate (incidence density)	New cases	Person‑time at risk	Speed of occurrence; units = per time
Point prevalence	Existing cases at a time point	Population at that point	Burden at a moment
Period prevalence	Existing at start + new during period	Average (mid‑period) population	Burden over an interval

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Measure

Numerator

Denominator

Interpretation

Incidence proportion (cumulative incidence, risk)

New cases in interval

Disease‑free population at risk at start

Probability over a stated period

Incidence rate (incidence density)

New cases

Person‑time at risk

Speed of occurrence; units = per time

Point prevalence

Existing cases at a time point

Population at that point

Burden at a moment

Period prevalence

Existing at start + new during period

Average (mid‑period) population

Burden over an interval