prevalence

The share of a population that has a given characteristic, such as a disease or a habit, at a given time, usually reported as a percentage.

Prevalence is the proportion of a population who have a specific characteristic in a given time period. The characteristic can be an illness, a condition or a behaviour such as using tobacco. It is usually reported as a percentage, or for rarer conditions as a number of cases per 10,000 or 100,000 people.

Researchers rarely count everyone. They survey a randomly selected sample, divide the number of people in the sample who have the characteristic by the total number sampled, and may apply statistical weights so that the sample matches the wider population.

Prevalence is different from incidence. Incidence counts new cases that appear during a period, while prevalence counts everyone who has the characteristic, however long ago it began. That makes prevalence the natural measure for a habit like smoking, where what matters is how many people are doing it now.

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