Question

5. The Nuchal Clarity Test is a test to detect the Down syndrome (trisomy). The test measures the liquid under the skin behind the neck of the fetus. The following table summarizes the data obtained from a sample of 1,000 pregnant women between 35 and 40 years old: T+ Down syndrome present Down syndrome absent Total (U+) (U_) 55 60 937 940 992 1000 w T_ Total (a) Calculate the false positive rate and the false negative rate of the test. (b) Calculate the sensitivity and specificity of the test.

Answer 1

(a) False positive rate= ratio between the number of negative events wrongly categorized as positive=55/992=0.0554

False negative rate= ratio between the number of positive events wrongly categorized as negative=3/8=0.375

(b) Sensitivity=True Positive/(True Positive+False Negative)=5/(5+3)=0.625

Specificity=True negative/(True negative+False positive)=937/(937+55)=0.9446