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The Polygraph and Lie Detection
FIGURE 7-2 Comparison of the false positive index and base rate for four values of the accuracy index (A) for a polygraph test protocol with threshold set to correctly identify 50 percent of deceptive examinees.
about 1,000 times higher when the base rate is 1 serious security risk in 1,000 than it is when the base rate is 1 in 2, or 50 percent.
The index is also affected, though less dramatically, by the accuracy of the test procedure: see Figure 7-2. (Appendix I presents the results of calculations of false positive indexes for various levels of accuracy, base rates, and thresholds for making a judgment of a positive test result.) With very low base rates, such as 1 in 1,000, the false positive index is quite large even for tests with fairly high accuracy indexes. For example, a test with an accuracy index of 0.90, if used to detect 80 percent of major security risks, would be expected to falsely judge about 200 innocent people as deceptive for each security risk correctly identified. Unfortunately, polygraph performance in field screening situations is highly unlikely to achieve an accuracy index of 0.90; consequently, the ratio of false positives to true positives is likely to be even higher than 200 when this level of sensitivity is used. Even if the test is set to a somewhat lower level of sensitivity, it is reasonable to expect that each spy or terrorist that might be correctly identified as deceptive by a polygraph test of the accuracy actually achieved in the field would be accompanied by at least hundreds of nondeceptive examinees mislabeled as deceptive. The spy or terrorist would be indistinguishable from these false positives by poly-