Prevalence  (synonym: prevalence proportion) is expressed as a percentage.

The prevalence measures the proportion of individuals in a population with a specific disease (or a specific characteristic) at a certain point of time (T0). The term prevalence pool is sometimes used to identify the subgroup of the population with the disease. Some individuals exit the prevalence pool by recovering or dying, some enter by developing disease. However prevalence is measured at a specific point in time (T0). The prevalence will range from 0 to 1 or 0% to 100% if expressed as a percentage. Since time is not measured in the denominator the prevalence is not a rate.

The prevalence reflects both disease incidence and disease duration. The higher the incidence and the longer the duration of a disease, the larger the amount of people with the disease at a specific point in time. If incidence rates and duration are stable over time, the prevalence and incidence rate are related in the following way : in which P is the prevalence, I the incidence rate and D the average disease duration.

If the prevalence is low, 1 – P tends to 1 and P / (1 – P) is almost equal to P.

Then the formula can be simplified as: The prevalence proportion is used to measure disease burden in a population. It applies more to administrative areas of public health than into the cause of disease. It is also used to describe characteristics or conditions other than diseases (vaccine coverage, prevalence of smokers, prevalence of blood groups, etc.).

## References

1. Rothman KJ; Epidemiology: an introduction. Oxford University Press 2002, p.40-44.
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