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How is an incidence proportion calculated?

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Know the investigations of researchers using genomic information to reconstruct the cause and transmission routes of the bubonic plague and the Black Death — epidemiology Researchers using genomic information to trace the transmission routes in past epidemics of plague.

In epidemiology, the incidence proportion—also known as cumulative incidence—quantifies the risk of an individual developing a specific disease or condition over a defined time period.12

To calculate the incidence proportion, you use the following formula:

$Incidence Proportion = \frac{Number of new cases during a given time period}{Total population at risk during the same period}$

For this calculation to be accurate, all individuals in the denominator must be at risk of the outcome and free of the disease at the start of the study period. Because this method requires complete follow-up of all participants, it is most effectively used in studies with short follow-up durations.12

For example, if 84 passengers on a cruise ship develop a gastrointestinal illness out of 2,318 total passengers, the incidence proportion is calculated as $84 / 2, 318$ , resulting in approximately 4 percent over that seven-day period.1