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Question 687
Statistics → Epidemiology
Classification
Question
SSBAQuestion Header
Statistics
Question Stem
Which formula correctly calculates the specificity of a diagnostic test?
Options
A
True positives / (true positives + false negatives)
B
True negatives / (true negatives + false positives)
C
True positives / (true positives + false positives)
D
True negatives / (true negatives + false negatives)
E
False positives / (false positives + true negatives)
Explanation
Specificity is the probability that a test is negative when the condition is absent. In the standard 2 × 2 diagnostic table it is calculated as true negatives divided by the sum of true negatives and false positives. It therefore describes the ability of the test to be correctly negative among people known not to have the disease.
Option Validity
A) This is the formula for sensitivity.
C) This is the formula for positive predictive value.
D) This is the formula for negative predictive value.
E) This gives the false-positive proportion among those without disease rather than specificity.
Further Reading
Saxena - MRCOG-1 Basic Sciences, Chapter 10: Epidemiology/Statistics, Printed p.287.