Attack rate

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In epidemiology, an attack rate is the cumulative incidence of infection in a group of people observed over a period of time during an epidemic, usually in relation to foodborne illness. Quantitatively, it is the number of exposed people who develop the disease divided by the total number of exposed people.

It is measured from the beginning of an outbreak to the end of the outbreak. The term should probably not be described as a rate because its time dimension is uncertain - in epidemiology, a rate requires a defined unit change (in this instance, time) over which the rate applies.[1] For this reason, it is often referred to as an attack ratio.

For instance, if there are 70 people taken ill out of 98 in an outbreak, the attack rate is

\frac{70}{98} \cong 0.714

or about 71.4%.


  1. ^ International Epidemiological Association (2001). A Dictionary of Epidemiology. (Last JM, editor) (4th ed. ed.). Oxford University Press. ISBN 978-0-19-514169-6.