cheapest of the perfect tests, and use it if the annual costs of the test itself and of spying between tests were less than the annual costs of spies with no screening. Unfortunately, all currently known screening tests are imperfect. A third parameter, P(a), represents the performance (accuracy) of screening program a for detecting spies and avoiding false accusations. Because polygraph screening programs involve more than just the polygraph test (for example, the effect of the interrogation depends on examinees’ perceptions of polygraph accuracy), P(a) depends on more than just the polygraph test alone, and may be different from the accuracy index (A) of the polygraph test procedure. Bayes’ theorem can be used to calculate the number of false positives and true positives as a function of policy and to select the appropriate threshold for labeling an employee as deceptive (or, more specifically, as a security risk or a spy), given the calculations of net costs.

To estimate the parameters for the model, one would need to use judgment (preferably informed by statistical evidence) to calculate the base rate of espionage and a plausible range of values. For example, the estimate of the probability that an employee is a spy might be based on the 139 known spies from 1940-1994 (Taylor and Snow, 1997) added to an estimate of the spies that were caught but not reported for security reasons, and the estimated number of spies who were not caught in this period, divided by the number of people working in that period with access to critical information. This probability would vary from agency to agency and over time.

The variable costs of a screening program are primarily labor and could be estimated from the number of cases done each year, multiplied by the average salaries paid to examiners and examinees for the time they spend in the screening process. Fixed costs might be estimated by some standard overhead amount or by a detailed costing. Alternatively, the total monetary costs might be estimated by taking the annual polygraph program budget and estimating the portion used in screening activities.

Chapter 5 is primarily concerned with assessing the accuracy of polygraph testing in various situations. Accuracy may depend on the testing procedure, the situation, and characteristics of examiners and examinees, as well as the base rate of espionage and the decision threshold selected for each decision point in a screening program. Historical data on performance is needed for estimating the likely numbers of false positives and false negatives, as well as a subjective assessment of the relationship of the historical data to the current context.

To evaluate the predicted consequences for each policy, it is necessary to frame the analysis by choosing a common perspective for all programs, which in this case would be a societal viewpoint, rather than that of a particular agency. The simplest way to combine outcomes in differ-



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