Evaluation of a diagnostic test on general patient samples is often done within the context of ongoing clinical care. This may be problematic if the test is incorporated into the diagnostic process for these patients. Unless special care is taken, other diagnostic findings (e.g., an image) may then influence the interpretation of the test results, or the test result itself may stimulate further investigation that uncovers the final diagnosis against which the test is then evaluated. These types of “contamination” have been extensively studied in relation to what is termed “verification bias” (see Begg and Greenes, 1983). They artificially increase the correlation between a test result and its diagnostic reference, also exaggerating the accuracy of the test relative to what would be seen in field application.
Manifestations of these issues in evaluations of polygraph testing are apparent. Laboratory researchers have the capacity to exercise good control over contamination threats to internal validity. But such research typically uses subjects who are not representative of those examined in the field and are under artificial, uniform, and extremely clear-cut conditions. Polygraph instrumentation and maintenance and examiner training and proficiency are typically well above field situations. Testing is undertaken concurrent with or immediately after the event of interest, so that no period of potential memory loss or emotional distancing intervenes.
Thus, laboratory evaluations that correctly mirror laboratory performance are apt to overestimate field performance. But field evaluations are also apt to overestimate field performance for several reasons. The polygraph counterpart to contamination of the diagnostic process by the test result has been discussed in Chapter 4. So has the counterpart to evaluating only those cases for which the true condition is definitively known. In addition, expectancies, particularly those of examiners, are readily contaminated in both field applications and evaluations of field performance. Polygraph examiners typically enter the examination with information that shapes their expectations about the likelihood that the examinee is guilty. That information can plausibly influence the conduct of the examination in ways that make the test act somewhat as a self-fulfilling prophecy, thus increasing the apparent correspondence between the test result and indicators of truth and giving an overly optimistic assessment of the actual criterion validity of the test procedure.
In view of the above issues, we believe that the range of accuracy indexes (A) estimated from the scientifically acceptable laboratory and field studies, with a midrange between 0.81 and 0.91, most likely over-states true polygraph accuracy in field settings involving specific-incident investigations. We remind the reader that these values of the accuracy index do not translate to percent correct: for any level of accuracy, per-