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The Polygraph and Lie Detection
situations for which both types of tests are applicable. Indeed, the concealed information tests we examined did exhibit higher median accuracy than the comparison question tests, though the observed difference did not attain conventional statistical significance. Specifically, the median accuracy index among 13 concealed information tests was 0.88, with an interquartile range from 0.85 to 0.96, while the corresponding median for 37 comparison question tests was 0.85, with an interquartile range from 0.83 to 0.90. (Two research reports did not fit either of these two test formats.) The arithmetic mean accuracies, and means weighted by sample size or inverse variance, were more similar than the reported medians. We regard the overall evidence regarding comparative accuracy of control question and concealed knowledge test formats as thus suggestive but far from conclusive.
Our data do not suggest that accuracy is associated with the size of the study samples, our ratings of the studies’ internal validity and their salience to the field, or the source of funding.2 We also examined the dates of the studies to see if research progress had tended to lead to improvements in accuracy. If anything, the trend ran against this hypothesis. (Appendix H presents figures summarizing the data on accuracy as a function of several of these other factors.)
It is important to emphasize that these data and their descriptive statistics represent the accuracy of polygraph tests under controlled laboratory conditions with naïve examinees untrained in countermeasures, when the consequences of being judged deceptive are not serious. We discuss below what accuracy might be under more realistic conditions.
Only seven polygraph field studies passed our minimal criteria for review. All involved examination of polygraph charts from law enforcement agencies’ or polygraph examiners’ case files in relation to the truth as determined by relatively reliable but nevertheless imperfect criteria, including confession by the subject or another party or apparently definitive evidence. The seven datasets include between 25 and 122 polygraph tests, with a median of 100 and a total of 582 tests. Figure 5-3 displays results in the same manner as in Figure 5-1. The accuracy index values (A) range from 0.711 to 0.999, with a median value of 0.89, which, given sampling and other variability, is statistically indistinguishable from the median of 0.86 for the 52 datasets from laboratory studies. There were no obvious relationships between values of A and characteristics of the studies. (Further discussion of these data appears in Appendix H.)
These results suggest that the average accuracy of polygraph tests examined in field research involving specific incident investigations is