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Prevalence & Incidence: Simple Definition

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  1. What is Incidence?
  2. What is Prevalence?

1. What is Incidence?

Incidence is the number of newly diagnosed cases of disease. For example, let’s say over the last year, five new cases of Ebola emerge out of a village population of 200 (where all people in the group were free of Ebola symptoms prior to this date). Then the incidence for this population is 0.025 (2,500 per 100,000 women-years of study).

Knowing the incidence of a disease in a population is extremely useful in health statistics, as it gives you any person’s probability of getting diagnosed with a disease during a certain period of time.

2. What is Prevalence?

Prevalence, in statistics, is defined as the proportion of a population which has a particular characteristic at a given time or during a specified time period. It is often reported as a percentage, or as the number of cases per 10,000 / 100,000.

For example, if 7 percent of people in Anchorage, Alaska had the flu on July 1, 2018 the prevalence of people in Anchorage with the flu, on July 1 2018, would be 7 percent.


New York State Department of Health (1999). Basic Statistics: About Incidence, Prevalence, Morbidity, and Mortality – Statistics Teaching Tools. Retrieved July 9, 2019 from:


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