Prevalence Rate Formula Calculator

About Prevalence

In epidemiology, prevalence (also known as Prevalence Rate) defines the proportion of a particular population that is affected by a disease, risk factor or other studied outcome. This can either be measured at a particular time or over a specified period of time.

Prevalence Rate (%) = New and pre-existing cases of disease during the same time period / Population size during the same time period x 100

Often confused with incidence, it is important to distinguish that prevalence includes all cases, new and pre-existing, in a population whilst incidence only measures new cases that develop the condition. In relation to the time period, prevalence may be classified as:

• Point prevalence – P (%) measured at a particular point in time, on a particular date.
• Period prevalence – P (%) measured over an interval of time.

A high prevalence of a disease within a population might result from either high incidence or a prolonged rate of survival without a cure or from both these factors.

A low prevalence might result from either low incidence of the condition, rapid mortality or rapid recovery or a combination of these.

The prevalence indicator is preferred to incidence when measuring chronic diseases which are characterised by a long duration and difficulty in pointing out the exact date of onset.

Prevalence is usually reported as a percentage, but may also be found as no out of 100 or as number of cases per 10,000 or 100,000 people.

References

Centers for Disease Control and Prevention: Principles of Epidemiology in Public Health Practice, Third Edition – An Introduction to Applied Epidemiology and Biostatistics.

Spronk I, Korevaar JC, Poos R, Davids R, Hilderink H, Schellevis FG, Verheij RA, Nielen MMJ. Calculating incidence rates and prevalence proportions: not as simple as it seems. BMC Public Health. 2019; 19(1):512.