Evidence-appraisal glossary
Prevalence
Prevalence is the proportion of a population that has a condition at a given time. It counts existing cases (both old and new) divided by everyone in the group. Because it reflects how common something is right now, it depends on both how often the condition arises and how long it lasts.
Also called: point prevalence, period prevalence.
Prevalence measures how widespread a condition is in a population at a point in time (point prevalence) or over a defined interval (period prevalence). It is calculated as the number of people who have the condition divided by the total number in the group, usually expressed as a percentage or per 1,000. Because it counts all existing cases, prevalence rises when a condition lasts longer, not only when new cases appear; a highly treatable illness that resolves quickly can have low prevalence even if many people catch it. When reading a study, check whether a figure describes prevalence (existing cases) or incidence (new cases), since mixing them distorts interpretation. Ask what time frame and population the estimate covers, and whether the sample represents that population. For example, if 200 of 10,000 surveyed adults report diagnosed diabetes, the prevalence is 2 percent. Prevalence is useful for planning services and describing burden, but it cannot by itself tell you a condition's risk or cause.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.