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.



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