Sensitivity is defined as the ability of a test to detect the presence of a disease in those who truly have the disease. It is calculated as the number of people with a disease who test positive (true positive) divided by the total number of people who have the disease (true positive+ false negative). In this case, sensitivity equals the number of babies born with trisomy 21 whose mothers tested positive (100) divided by the total number of babies born with trisomy 21. This yields 100/200 = 50% (not a very sensitive test). 40% corresponds to the prevalence of the disease in the tested population, which in this case equals the total number of babies with Down's syndrome (true positive + false negative = 100 + 100= 200) divided by the total number of people tested (500). This yields 200/500= 40%. 67% corresponds to the positive predictive value of the test, which equals the number of babies with Down's whose mothers test positive (true positives = 100) divided by the total number of mothers testing positive(true positive + false positive = 150). This yields 100/150 = 67%. 71% corresponds to the negative predictive value of the test, which equals the number of normal babies whose mothers tested negative (250) divided by the total number of people testing negative (350). This yields 250/350 = 71%.