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The Polygraph and Lie Detection
(or their supervisors). In medical charts, for example, observations of the variables critical to immediate patient care are generally accurate while others perhaps needed later for retrospective research are often omitted or present only by implication. Polygraph research would present a similar situation.
We appreciate the inherent difficulty of determining the truth for observational polygraph field studies. Although we applaud the labor of those investigators who have undertaken such studies, we are unable to place a great deal of faith in this small body of work, especially regarding its implications for screening. We found only one field study of polygraph screening with verifiable outcome data relevant to assessing accuracy; its results and limitations are discussed in Chapter 5. The annual reports that polygraph programs provide to Congress do not provide a basis for assessing the accuracy of polygraph testing, as we have discussed.
We found no specific-incident field investigations at the higher levels of the research hierarchy outlined above. The literature revealed no experiments and no cohort or case-control studies that were prospectively designed and implemented. The best criminal field investigations we reviewed were observational case-control studies using data on truth obtained retrospectively from administrative databases. In these studies, the past polygraph judgments (or reevaluations of past polygraph records) with respect to individuals whose deceptiveness or nondeceptiveness had subsequently been established were reviewed, tabulated, and compared. This case-control approach is an observational research design of intermediate strength, weakened in most of these studies by heterogeneity of polygraph procedure; lack of prospective, research-oriented data collection; and the probable contamination of sample selection by the polygraph result. Data were generally not provided on whether confessions occurred during the polygraph examination or subsequently as a direct consequence of being judged deceptive on the polygraph examination. Neither were data provided on the extent to which a suspect’s polygraph results led an investigation to be redirected, leading to the determination of the truth. Both these outcomes of the polygraph examination are good for law enforcement, but they lead to overestimates of polygraph accuracy.
Although we excluded studies that lack independent evidence of truth, field study procedures still tend to overestimate the accuracy of the polygraph. The problem, in technical terms, is that these studies use the probabilities of past truthful or deceptive polygraph outcomes among subsets of examinees later proven to be truthful or deceptive to estimate the probabilities of future polygraph outcomes among all examinees, including those for whom the truth cannot be independently established.