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Reference.org
Prevalence
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<p>In <a href="/facts/Epidemiology/wgTmxoMA">epidemiology</a>, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number of people found to have the condition with the total number of people studied and is usually expressed as a fraction, a percentage, or the number of cases per 10,000 or 100,000 people. Prevalence is most often used in <a href="/facts/Questionnaire/15o4lgqm">questionnaire</a> studies. </p>