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148 Statistics in service to the nationFIGURE 13.1Sensitivity and false positive rates in 52 laboratory datasets on polygraphvalidity. Reprinted with permission from The Polygraph and Lie Detectionby the National Academy of Sciences (NRC, 2003). Courtesy of the NationalAcademies Press, Washington, DC.13.7 The accuracy of the polygraphIn 2000, I was asked to chair yet another NRC committee on the accuracy ofpolygraph evidence in the aftermath of the “Wen-Ho Lee affair” at Los AlamosNational Laboratory, and the study was in response to a congressional request.My principal qualification for the job, beyond my broad statistical backgroundand my research and writing on forensic topics, was ignorance — I had neverread a study on the polygraph, nor had I been subjected to a polygraph exam.I will share with you just one figure from the committee’s report (NRC, 2003).Figure 13.1 takes the form of a receiver operating characteristic plot (orROC curve plot) that is just a “scatterplot” showing through connected linesthe sensitivity and specificity figures derived from each of the 52 laboratorystudies that met the committee’s minimal quality criteria. I like to refer to thisas our “show me the data” plot. Each study has its own ROC curve, pointsconnected by dotted lines. Each point comes from a 2 × 2contingencytable.You can clearly see why we concluded that the polygraph is better than chancebut far from perfect! Further, there are two smooth curves on the graph representingthe accuracy scores encompassed by something like the interquartilerange of the experimental results. (Since there does not exist a natural definitionfor quartiles for multivariate data of this nature, the committee first