This slide show illustrates how to calculate false positive and false negative polygraph test results.
There are 4 possible situations in a polygraph test. The test can indicate that the subject lied when in fact the subject told the truth. This is known as a false positive. The test can indicate that the subject told the truth when in fact the subject lied. This is know as a false negative. The other two cases occur when the test gives an accurate result: the test accurately identifies a truth-teller or a liar. The slide show depicts these situations and gives the probabilities for each.
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There are two possibilities for an inaccurate polygraph test result. The first possibility is if the polygraph indicates that the subject lied when he/she actually did not lie. This is known as a false positive. The second possibility is a polygraph that indicates the subject told the truth when he/she actually lied. This is known as a false negative.