false positive index, these determinants are by no means equally important. Calculation of the index for diagnostic tests at various levels of accuracy, using various thresholds, and with a variety of base rates shows clearly that base rate is by far the most important of these factors. Figure 7-1 shows the index as a function of the base rate of positive (e.g., deceptive) cases for three thresholds for a diagnostic test with A = 0.80. It illustrates clearly that the base rate makes more difference than the threshold across the range of thresholds presented. Figure 7-2 shows the index as a function of accuracy with the threshold held constant so that the diagnostic test’s sensitivity (percent of deceptive individuals correctly identified) is 50 percent. It illustrates clearly that base rate makes more difference than the level of accuracy across the range of A values represented.

Figures 7-1 and 7-2 show that the tradeoffs involved in relying on a diagnostic test such as the polygraph, represented by the false positive index values on the vertical axis, are sharply different in situations with high base rates typical of event-specific investigations, when all examinees are identified as likely suspects, and the base rate is usually above 10 percent, than in security screening contexts, when the base rate is normally very low for the most serious infractions. The false positive index is

FIGURE 7-1 Comparison of the false positive index and base rate for three sensitivity values of a polygraph test protocol with an accuracy index (A) of 0.80.

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