Polygraph lie-detector machines are commonly used in criminal investigations. the device measures nervous excitement, operating on the idea that if a person is telling the truth they will remain calm. the american polygraph association claims that polygraphs accurately identify liars 90% of the time. in other words, if a person lied the test results will be positive 90% of the time. although the polygraph test is good at identifying liars, there is a 50% chance that the polygraph test will say a truthful person is lying. a police force wants to predict the accuracy of lie-detector tests for 1,000 suspected criminals. assume that 700 per 1,000 suspected criminals tell the truth during polygraph tests. lie truth row totals test result: negative p test result: postive q column totals 1,000 barber, n. (2013). do lie detectors work? psychology today . what is the probability that a person is telling the truth given that the lie detector test has a positive result?