Prevalence

How often or how frequently a disease or condition occurs in a group of people. Prevalence is calculated by dividing the number of people who have the disease or condition by the total number of people in the group.

For example, health leaders in a community were concerned that some women were not receiving health care and therefore did not know they had serious problems. The health leaders began a breast cancer screening program. Women were encouraged to come into a community clinic and have breast examinations. In this example, out of 1,000 women, 20 had breast cancer. Therefore, the prevalence of breast cancer in women who undergo screening for breast cancer is 20/1000, or 2 percent.

Related Terms

Positive Predictive Value

Indicates the likelihood that a person with a positive test result would actually have the condition for which the test is used. The higher the value of the positive predictive value (for example, 90 percent would be considered a high value), the more useful the test is for predicting that the person has the condition. Read more.

Pretest Probability

The probability that a person has a particular disease before any test results are obtained. The pretest probability for large groups of people (such as the population of a city) is the same as the prevalence of the disease in that group. Read more.

Prior Probability

The probability that a person has a particular disease before any test results are obtained. The prior probability for large groups of people (such as the population of a city) is the same as the prevalence of the disease in that group. Read more.

Risk

A way of expressing the chance that something will happen. It is a measure of the association between exposure to something and what happens (the outcome). Risk is the same as probability, but it usually is used to describe the probability of an adverse event. It is the rate of events (such as breast cancer) in the total population of people who could have the event (such as women of a certain age). Read more.