what is positive predictive value
We can now define positive and negative predictive value as follows: Positive predictive value = a / (a+b). Negative predictive value = d / (c+d). Post-test probability of disease given a positive test = a / (a+b). Post-test probability of disease given a negative test = c / (c+d). Notice that we are now using the rows instead of columns, as for sensitivity and specificity.