Population at Risk

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The term "population at risk" defines the denominator for the calculation of rates of incidences and prevalence. It alludes to the number of persons potentially capable of experiencing the event or outcome of interest. The number or persons who actually experience the event make up the numerator of the rate. For rates to be valid and meaningful, the population at risk must be known accurately. However, many vital statistical rates are calculated on an annual basis, and for these rates the midyear population, or an approximation based on extrapolation from a recent set of census data, is often used for the population at risk. As rates incorporate an element of time, the denominator is often expressed in person-time rather than as the population at risk.

John M. Last

(see also: Epidemiology; Rates; Rates: Adjusted )


Rothman, K. J., and Greenland, S., eds. (1998). Modern Epidemiology, 2nd edition. Philadelphia, PA: Lippincott-Raven.

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Population at Risk

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Population at Risk