3. Incidence

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2. Measurements of incidence

Incidence can be measured as a proportion or as a rate. Measured as a proportion, it quantifies the risk of an occurrence in a given time period. Measured as a rate, it quantifies the number of new cases in a population over time. Thus, to calculate incidence, three elements must be defined: (1) the number of new cases, (2) the population at risk, and (3) the time period.