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The Polygraph and Lie Detection
different from its intended target. A measurement process is considered valid if it measures what it is supposed to measure. As with reliability, there are several aspects to validity. It is particularly important for the committee’s work to distinguish between the empirical concept of criterion validity, or accuracy, and the theoretical concept of construct validity.
Criterion Validity (Accuracy)
Criterion validity refers to how well a measure, such as the classification of polygraph test results as indicating deception or nondeception, matches a phenomenon that the test is intended to capture, such as the actual deceptiveness or truthfulness of examinees on the relevant questions in the test. When the test precedes the criterion event, the term predictive validity is used; criterion validity is the more general term that applies even when the criterion event precedes the test, as it normally does with the polygraph. The term “’accuracy’’ is often used as a nontechnical synonym for criterion validity, and it is used in that way in this report. Polygraph accuracy is the extent to which test results correspond to truth with actual examinees. The proportion of correct judgments made by a polygraph examiner is a commonly used measure of accuracy for the polygraph test. (We discuss the shortcomings of this measure of accuracy and propose a more appropriate one below.)
Individual polygraph validation studies typically include accuracy measures that apply to the specific population that was tested. Evidence of accuracy becomes more general to the extent that test results are strongly and distinctively associated with truthfulness or deception in a variety of populations. Populations of interest include those containing high proportions of individuals who can be presumed to be deceptive on the critical questions (e.g., criminal suspects); those with low proportions of such people (e.g., nuclear scientists, intelligence agents); special populations that may be likely to show false negative results (e.g., people who want to deceive the examiner and who use countermeasures to try to “beat” the test); and populations that may be likely to show false positive results (e.g., truthful people who are highly anxious about the test). The same is true for test situations. Evidence of accuracy becomes more general as test results correspond with actual truthfulness or deceptiveness across situations (e.g., in criminal investigations, in employee security screening, and so forth). It is possible for a test such as the polygraph to be more accurate in some situations (e.g., criminal investigations) than in others (e.g., employee screening).