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Question 689
Statistics → Epidemiology
Classification
Question
SSBAQuestion Header
Statistics
Question Stem
A diagnostic test produces 80 true-positive results and 20 false-positive results. What is its positive predictive value in this sample?
Options
A
20%
B
40%
C
60%
D
80%
E
100%
Explanation
Positive predictive value is the probability that a person with a positive test actually has the condition. It is calculated as true positives divided by true positives plus false positives. Therefore PPV = 80/(80 + 20) = 80/100 = 80%.
Option Validity
A) 20% represents the proportion of false positives among all positive tests in this example.
B) The supplied values do not produce a PPV of 40%.
C) The supplied values do not produce a PPV of 60%.
E) A PPV of 100% would require no false-positive results.
Further Reading
Saxena - MRCOG-1 Basic Sciences, Chapter 10: Epidemiology/Statistics, Printed pp.287–288.