The performance of a screening test is measured by its predictive values. Predictive value positive (PV+), or yield of a screening program, is the probability that a person is actually affected  given that he or she tests positive. Predictive value negative (PV-) is the probability that an individual is actually unaffected given a negative screening test. Contrary to sensitivity and specificity, the predictive values depend on the prevalence, the higher the prevalence the higher the PV+  and the lower the PV-. The percentage of correct classification (PCC), is the ability of the screening program to correctly classify individuals either affected or unaffected. The methods of calculating these measures are given in last slide