Work out sensitivity and specificity instantly with clear inputs, formula shown and shareable results.
Sensitivity is the fraction of diseased people the test catches and specificity the fraction of healthy people it clears. Predictive values answer the patient's question instead, and they depend heavily on prevalence.
Sensitivity and specificity
Sens = TP/(TP + FN), Spec = TN/(TN + FP)
Predictive values
PPV = TP/(TP + FP), NPV = TN/(TN + FN)
Sensitivity is 90% and specificity 95%, but the positive predictive value is only 66.7%.
Because the healthy group is nine times larger here, so its 5% false-positive rate contributes many of the positives.