Chapter 12, Problem 86E

Polygraphs Lie detectors are controversial instruments, barred from use as evidence in many courts. Nonetheless, many employers use lie detector screening as part of their hiring process in the hope that they can avoid hiring people who might be dishonest. There has been some research, but no agreement, about the reliability of polygraph tests. Based on this research, suppose that a polygraph can detect 65% of lies, but incorrectly identifies 15% of true statements as lies.

A certain company believes that 95% of its job applicants are trustworthy. The company gives everyone a polygraph test, asking, “Have you ever stolen anything from your place of work?” Naturally, all the applicants answer “No,” but the polygraph identifies some of those answers as lies, making the person ineligible for a job. What’s the probability that a job applicant rejected under suspicion of dishonesty was actually trustworthy?