# The Polygraph and Lie Detection(2003)

## Chapter: Appendix I: False Positive Index Values for Polygraph Testing

« Previous: Appendix H: Quantitative Assessment of Polygraph Test Accuracy
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Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
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## Appendix I False Positive Index Values for Polygraph Testing

Table I-1 illustrates the effects of accuracy and thresholds on the false positive index, with four illustrative base rates (the false positive index is the number of false positive test results for each true positive test result). It shows that increasing test accuracy makes for more attractive tradeoffs in using the test. For example, it shows that that for any base rate, if the threshold is set so as to correctly detect 50 percent of truly positive cases, or major security risks (shown in Table I-1B), a diagnostic with A = 0.80 has a false-positive index of about three times that of a diagnostic with A = 0.90; a diagnostic with A = 0.70 has an index of about six times that of a test with A = 0.90; and a diagnostic with A = 0.60 has an index of about eight times that of a test with A = 0.90. These ratios vary somewhat with the threshold selected, but they illustrate how much difference it would make if a high value of A could be achieved for field polygraph testing. If the diagnosis of deception could reach a level of A = 0.90, testing would produce much more attractive tradeoffs between false positives and false negatives than it has at lower levels of A. Nevertheless, if the proportion of major security risks in the population being screened is equal to or less than 1 in 1,000, it is reasonable to expect even with optimistic assessments of polygraph test accuracy that each spy or terrorist that might be correctly identified as deceptive would be accompanied by at least hundreds of nondeceptive examinees mislabeled as deceptive, from whom the spy or terrorist would be indistinguishable by polygraph test result. The possibility that deceptive examinees may use countermeasures makes this tradeoff even less attractive.

Figures I-1 through I-4 enable readers to derive values of the false

Page 355
Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
×

TABLE I-1A Values of the False Positive Index with Decision Thresholds Set for 80 Percent Sensitivity

 Base rate A = 0.90 A = 0.80 A = 0.70 A = 0.60 0.001 208 452 634 741 0.01 21 45 63 73 0.10 1.9 4.1 5.7 6.7 0.50 0.21 0.45 0.63 0.74

TABLE I-1B Values of the False Positive Index with Decision Thresholds Set for 50 Percent Sensitivity

 Base rate A = 0.90 A = 0.80 A = 0.70 A = 0.60 0.001 70 232 411 545 0.01 7.0 23 41 54 0.10 0.63 2.1 3.7 4.9 0.50 0.07 0.23 0.41 0.55

TABLE I-1C Values of the False Positive Index with Decision Thresholds Set for 20 Percent Sensitivity

 Base rate A = 0.90 A = 0.80 A = 0.70 A = 0.60 0.001 20 104 240 370 0.01 2.0 10 24 37 0.10 0.18 0.94 2.2 3.3 0.50 0.02 0.10 0.24 0.37

positive index (FPI) from assumptions about the base rate of deceptive examinees in a population to be given polygraph tests, the level of accuracy achieved by the polygraph, and the decision threshold, defined in terms of the sensitivity, or proportion of deceptive individuals to be identified correctly. The figures show values for accuracy rates (A) of 0.90, 0.80, 0.70, and 0.60 and sensitivities of 80, 50, and 20 percent. The figures are based on the binormal, equivariance model and are presented on logarithmic scales to make it easier to get accurate readings for very low base rates than is possible with standard scales such as presented in Figures 7-1 and 7-2.

Page 356
Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
×

FIGURE I-1 False positive index values as a function of base rate of deception for a diagnostic procedure with an accuracy index (A) of 0.90 and threshold values achieving sensitivities of 80 percent, 50 percent, and 20 percent.

FIGURE I-2 False positive index values as a function of base rate of deception for a diagnostic procedure with an accuracy index (A) 0.80 and threshold values achieving sensitivities of 80 percent, 50 percent, and 20 percent.

Page 357
Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
×

FIGURE I-4 False positive index values as a function of base rate of deception for a diagnostic procedure with an accuracy index (A) of 0.60 and threshold values achieving sensitivities 80 percent, 50 percent, and 20 percent.

FIGURE I-3 False positive index values as a function of base rate of deception for a diagnostic procedure with an accuracy index (A) of 0.70 and threshold values achieving sensitivities of 80 percent, 50 percent, and 20 percent.

Page 354
Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
×
Page 355
Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
×
Page 356
Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
×
Page 357
Suggested Citation:"Appendix I: False Positive Index Values for Polygraph Testing." National Research Council. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. doi: 10.17226/10420.
×
Next: Appendix J: Decision Analysis of Polygraph Security Screening »
The Polygraph and Lie Detection Get This Book
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The polygraph, often portrayed as a magic mind-reading machine, is still controversial among experts, who continue heated debates about its validity as a lie-detecting device. As the nation takes a fresh look at ways to enhance its security, can the polygraph be considered a useful tool?

The Polygraph and Lie Detection puts the polygraph itself to the test, reviewing and analyzing data about its use in criminal investigation, employment screening, and counter-intelligence.

The book looks at:

• The theory of how the polygraph works and evidence about how deceptiveness—and other psychological conditions—affect the physiological responses that the polygraph measures.
• Empirical evidence on the performance of the polygraph and the success of subjects' countermeasures.
• The actual use of the polygraph in the arena of national security, including its role in deterring threats to security.

The book addresses the difficulties of measuring polygraph accuracy, the usefulness of the technique for aiding interrogation and for deterrence, and includes potential alternatives—such as voice-stress analysis and brain measurement techniques.

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