# pretest probability

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## prevalence

Epidemiology(1) The number of people with a specific condition or attribute at a specified time divided by the total number of people in the population.

(2) The number or proportion of cases, events or conditions in a given population.

Statistics

A term defined in the context of a 4-cell diagnostic matrix (2 X 2 table) as the amount of people with a disease, X, relative to a population.

Veterinary medicine

(1) A clinical estimate of the probability that an animal has a given disease, based on current knowledge (e.g., by history of physical exam) before diagnostic testing.

(2) As defined in a population, the probability at a specific point in time that an animal randomly selected from a group will have a particular condition, which is equivalent to the proportion of individuals in the group that have the disease. Group prevalence is calculated by dividing the number of individuals in a group that have a disease by the total number of individuals in the group at risk of the disease. Prevalence is a good measure of the amount of a chronic, low-mortality disease in a population, but is not of the amount of short duration or high-fatality disease. Prevalence is often established by cross-sectional surveys.

## pretest probability

An estimate of a patient's likelihood of illness. It is based on or derived from the prevalence of disease in a community.

See also: probability

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