reliability is the extent to which the same measurement procedure (with the polygraph, this includes the examiner, the test format, and the equipment) used to examine the same subject for the same purpose yields the same result on repetition.1Inter-rater reliability is the extent to which different examiners would draw the same conclusions about a given subject at a given time for a given examination. In practice and in the literature we have considered, discussions of inter-rater reliability have focused almost exclusively on the repeatability of chart scoring across human or computer raters. Inter-rater reliability has been a critical issue in some celebrated practical uses of the polygraph. (Appendix C describes the use of the polygraph in investigations of Wen Ho Lee for espionage or other security violations; part of the story concerns differing interpretations of the results of a 1998 polygraph ordered by the U.S. Department of Energy.)

There is also potentially large variability in ways an examination is conducted: which questions are asked, how they are asked, and the general atmosphere of the examination. This variability can in principle seriously threaten test-retest reliability to the extent that polygraph examiners have latitude in asking questions.2 Reliability across examinees is another important component of overall test reliability. For example, two examinees may have engaged in the same behaviors and may give the same answers to the same test questions, but due to different interpretations of a question, may have differing beliefs about the truthfulness of their responses and so produce different polygraph readings.

Internal consistency is another aspect of reliability. For example, a polygraph test may be judged to indicate deception mainly because of a strong physiological response to a single relevant question. If the examinee shows similar responses to other relevant questions about the same event or piece of information, the test is internally consistent.

Reliability is usually defined as a property of a measure as used on a particular population of people or events being measured. If the polygraph is to be applied in standard ways across a range of people and situations, it is desirable that measures be reliable across the range of people and situations being measured—whether subjects and examiners are calm or nervous, alert or sleepy, relaxed or under time pressure, male or female, from the same or different cultural backgrounds, in the laboratory or in the field, etc.

Accuracy and Validity

Scientific inference requires measures that exhibit strong reliability. However, a highly reliable test has little use if it is measuring something

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