The nuchal translucency test is a special ultrasound scan which is widely used a
ID: 3070541 • Letter: T
Question
The nuchal translucency test is a special ultrasound scan which is widely used as a screening test for Down's syndrome in early pregnancy. The test measures the fluid under the skin at the back of the baby's neck and can be used to determine the risk of having a baby with Down's syndrome. The following table gives the test results for a sample of 1,000 pregnant women, with the age between 35 and 40:
(a) Compute the false positive rate and the false negative rate.
(b) Compute the sensitivity and the specicity of the test.
Explanation / Answer
Answer:
The nuchal translucency test is a special ultrasound scan
which is widely used as a screening test for Down's syndrome in early preg-
nancy. The test measures the
uid under the skin at the back of the baby's
neck and can be used to determine the risk of having a baby with Down's
syndrome. The following table gives the test results for a sample of 1,000
pregnant women, with the age between 35 and 40:
Down Syndrome Baby Normal Baby Total
Test + 5 55 60
Test - 3 937 940
Total 8 992 1000
DownSyndrome Baby
Normal Baby
Total
Test+
5
55
60
Test-
3
937
940
Total
8
992
1000
(a) Compute the false positive rate and the false negative rate.
false positive rate = 55/992 =0.055
false negative rate = 3/8 =0.375
(b) Compute the sensitivity and the specificity of the test.
Sensitivity = 5/8 =0.625
Specificity = 937/992 = 0.945
DownSyndrome Baby
Normal Baby
Total
Test+
5
55
60
Test-
3
937
940
Total
8
992
1000
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