Polygraph Testing: False Positives and False Negatives

Copyright 2002 David A. Wheeler, Ph.D., PHR, CMT NoStress@mail.com   Last updated 06/30/05 09:42 PM

File: polygraph.htm

Employee polygraph protection act 1988

Example

False negatives=30%
False positives=40%

Screen 1000 people
100 are liars
    Test will say 70 are lying, 30 are telling the truth
900  tell the truth
    Test will say that 540 are telling truth and 360 are lying

430 were caught  by the polygraph

but 70/430 were really liars


Drug Test Example

Screen 1000 people

100 actual drug users

False Positive: 10%
False Negative:1%

  Test says drugs Test says no drugs
100 Users 99 1
900 non Users 90 810
Total: 189 811

Retest the positives with more expensive test

False Negative 0.1%
False Positive 1%

 
  Test says drugs Test says no drugs
99 Users 98.911 .099
90 non Users .9 89.1
Total:    



 

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