[Dictionary] Negative Predictive Value (NPV)

Negative predictive value (NPV) is the proportion of all negative test results that are true negatives: NPV = TN / (TN + FN), calculated within the test-negative row of a diagnostic accuracy 2x2 table. It answers 'given that a patient tested negative, how likely is it the condition is genuinely absent' -- the mirror statistic to positive predictive value (PPV), which is calculated within the test-positive row of the same table. Like PPV, NPV depends on the prevalence of the condition in the population tested, but moves in the opposite direction: NPV rises as prevalence falls, while PPV falls.