The federal agencies that use the polygraph for screening do not collect data on admissions and confessions in a form that allows these field tests to be used to assess polygraph validity.
There is no direct scientific evidence assessing the value of the polygraph as a deterrent, as a way to elicit admissions and confessions, or as a means of supporting public confidence. The limited scientific evidence does support the idea that these effects will occur when examinees (and the public) perceive that there is a high likelihood of deception being detected and that the costs of being judged deceptive are substantial.
For the purposes of assessing accuracy, or criterion validity, it is appropriate to treat the polygraph as a diagnostic test and to apply scientific methods based on the theory of signal detection that have been developed for measuring the accuracy of such tests.
Diagnostic test performance depends on both the accuracy of the test, which is an attribute of the test itself, and the threshold value selected for declaring a test result positive.
There is little awareness in the polygraph literature and less in U.S. polygraph practice of the concept that false positives can be traded off against false negatives by adjusting the threshold for declaring that a chart indicates deception. We have seen indications that practitioners implicitly adjust thresholds to reflect perceived organizational priorities, but may not be fully aware of doing so. Explicit awareness of the concept of the threshold and appropriate policies for adjusting it to reflect the costs of different kinds of error would eliminate a major source of uncontrolled variation in polygraph test results.
The accuracy of the polygraph is appropriately summarized by the accuracy index A, as defined in the theory of signal detection. To estimate the accuracy of the polygraph, it is appropriate to calculate values for this index for the validation studies that meet standards of scientific acceptability and to consider whether these values are systematically related to other factors, such as populations of examinees, characteristics of individual examinees or examiners, relationships established in the interview, testing methods, and the use of countermeasures.