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A polygraph (popularly referred to as a lie detector) is an instrument that measures and records several physiological responses such as blood pressure, pulse, respiration, breathing rhythms, body temperature and skin conductivity while the subject is asked and answers a series of questions, on the theory that false answers will produce distinctive measurements. The polygraph measures physiological changes caused by the sympathetic nervous system during questioning.
Today, polygraph examiners use two types of instrumentation: analog and computerized. In the United States, most examiners now use computerized instrumentation.
A typical polygraph test starts with a pre-test interview to gain some preliminary information which will later be used for “Control Questions”, or CQ. Then the tester will explain how the polygraph is supposed to work, emphasizing that it can detect lies and that it is important to answer truthfully. Then a “stim test” is often conducted: the subject is asked to deliberately lie and then the tester reports that he was able to detect this lie. Then the actual test starts. Some of the questions asked are “Irrelevant” or IR (”Is your name John Doe?”), others are “probable-lie” Control Questions that most people will lie about (”Have you ever stolen money?”) and the remainder are the “Relevant Questions “, or RQ, that the tester is really interested in. The different types of questions alternate. The test is passed if the physiological responses during the probable-lie control questions (CQ) are larger than those during the relevant questions (RQ). If this is not the case, the tester attempts to elicit admissions during a post-test interview, for example, “Your situation will only get worse if we don’t clear this up”.
Criticisms have been given regarding the validity of the administration of the Control Questions test (CQT). The CQT may be vulnerable to being conducted in an interrogation-like fashion. This kind of interrogation style would elicit a nervous response from innocent and guilty suspects alike.
Numerous studies of polygraph validity have achieved rates of 80-95% for the kinds of tests used with specific issues, such as allegations in criminal cases. Validity of polygraph remains controversial. Despite claims of 90-95% reliability, critics charge that rather than a “test”, the method amounts to an inherently unstandardizable interrogation technique whose accuracy cannot be established.
Source: http://en.wikipedia.org/wiki/Polygraph
