Sensitivity	Proportion of diseased who have positive test results	True positives / total diseased a / (a + c)
Specificity	Proportion of nondiseased who have negative test results	True negatives / total nondiseased d / (b + d)
Positive predictive value (PPV)	Proportion with positive test result who actually have the disease	True positives / total test positive a / (a + b)
Negative predictive value (NPV)	Proportion with negative test result who really do not have the disease	True negatives / total test negative d / (c + d)