examination of the data, as well as modifications to the software for classification cutoffs. The instrumentation used was also a possible problem in this study, particularly for the CPS algorithm. Data were collected with the Axciton instrument that records a hybrid of skin conductance and skin resistance. The CPS algorithm relies on true skin conductance and the data recorded with the Stoelting instrument. The CPS algorithm was unable to process the Axciton proprietary data and was provided with the text format, in which there was also a possibility of error in rounding the onsets of the questions with further negative effect on the CPS performance. The other algorithms performed very similarly, which is not surprising because they were developed on data collected with Axciton instruments and in most cases with very similar databases.

IMPLICATIONS FOR TES

JHU-APL is currently working on a beta-test version of PolyScore 5.2 that has prototype algorithms for scoring screening test formats such as TES and relevant/irrelevant formats. The current version of the TES-format algorithm uses the same features as the ZCT/MGQT–format algorithm, but this may change. Polygraph examiners review each chart in a TES separately; PolyScore analyzes them together. We are not aware of other scoring algorithms for the TES format.

Table F-4 reports very preliminary results of the TES algorithm provided to us by JHU-APL. The current difficulty in developing this algorithm is the overall small number of deceptive cases. As a result, they are giving up the power to detect (that is, keeping the sensitivity of the test at lower levels) in order to keep the false positive rates lower, in effect changing the base rate assumptions. These data indicate that sensitivity of 70 percent may be attained in conjunction with 99 percent specificity (1 percent false positive rate). JHU-APL believes these numbers can be im-

TABLE F-4 Preliminary TES Results

Type of Analysis

Total Number

Inc

Corr

TN

FP

FN

TP

Binarya

716

0

707

692

4

5

15

Ternary

524

192

520

510

3

1

10

NOTES: Inc, inconclusive; Corr, correct; TN, true negative; FP, false positive; FN, false negative; TP, true positive.

aInconclusives forced to deceptive, nondeceptive.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement