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A polygraph (commonly referred to as a lie detector) is an instrument that measures and records several physiological responses such as blood pressure, pulse, respiration and skin conductivity while the subject is asked and answers a series of questions, on the basis that false answers will produce distinctive measurements. The polygraph measures physiological changes caused by the sympathetic nervous system during questioning. Within the U.S. federal government[clarify], a polygraph examination is also referred to as a psychophysiological detection of deception (PDD) examination. Several other technologies are also used in the field of lie detection, but the polygraph is the most famous.