Sampling bias is a potential problem in polygraph validity studies in which posttest confessions are used to establish ground truth because this criterion is not independent of the polygraph test. In the present study, criterion evidence was sought from polygraph office records and from independent police files for all 402 control question tests (CQTs) conducted during a 5-year period by federal police examiners in a major Canadian city. Based on blind scoring of the charts, the hit rate for criterion innocent subjects (65% of whom were verified by independent sources) was 55%; for guilty subjects (of whom only 2% were verified independently), the hit rate was 98%. Although the estimate for innocent subjects is tenable given the characteristics of the sample on which it is based, the estimate for the guilty subsample is not. Some alternatives to confession studies for evaluating the accuracy of the CQT with guilty subjects are discussed. The polygraph examination procedure used most frequently in criminal investigations is the control question test (CQT; cf. Barland & Raskin, 1973; Reid & Inbau, 1977), a procedure in which physiological responses to specific crime-relevant ques- tions are compared with responses to control questions dealing with the subject's prior history of wrongdoing. The CQT relies on the assumption that guilty subjects should be more con- cerrted with and hence more reactive to the relevant questions, whereas innocent subjects should be more disturbed by the more broadly incriminatory control questions. This assump- tion is without a sound theoretical basis (Ben-Shakhar, Bar-Hil- lel, & Lieblich, 1986; Kleinmuntz & Szucko, 1982; Lykken, 1978,1981), however, and therefore carefully controlled empiri- cal research is needed to establish the validity of the CQT. Ideally, an estimate of CQT validity would be derived from a representative sample of real-life examinations in which an un- ambiguous criterion of ground truth was available. Laboratory studies, although advantageous from the standpoint o fcriterion certainty, have generally failed to recreate the motivational and emotional characteristics of a real-life polygraph examination