Unless a polygraph test has perfect validity, a large number of false positive identifications can occur when the device is used to test for a behavior that has a low incidence in the general population.

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Quizplus · Accepted Answer

This is because the lower the incidence of the behavior in the general population, the higher the proportion of non-offenders (or non-behaviors) in the population. Even a small false positive rate (indicating someone has the behavior when they do not) can result in a large number of false identifications due to the large number of non-offenders being tested.

Unless a Polygraph Test Has Perfect Validity, a Large Number