Prevalence is all patients with disease divided by all patients tested. This is also termed the "prior probability."
prevalence =
= (a + c) / (a + b + c + d)
where:
• a + c = true positives + false negatives = all people with disease
• (a + b + c + d) = sum of (true positives, false positives, false negatives, true negatives) = total population
