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? Group of answer choices 350 out of 700= 0.50 350 out of 1,000 = 0.35 350 out of 620 = 0.56 350 out of 380 = 0.92
1 answer:
Answer:
0.5645
This means a person telling the truth is flagged as a liar 56.45% of the time. No, Polygraph lie detector machines do not work as this is too much a statistic for the innocent.
Step-by-step explanation:
Liars are identified 90% of the time.
Truth tellers are identified as truth tellers 50% of the time.
Out of 1000 Criminals, 700 criminals are telling the truth
Meaning 300 criminals are lying
Of the 700 truth tellers, the machine gets
it right 50% of the time
Meaning 350 positive results and 350 negative results.
Of the 300 liars, the machine gives a positive result 90% of the time.
Positive result = 0.9 × 300 = 270
Negative result = 0.1 × 300 = 30
The table of results is presented in the second attached image
The probability that a person is telling the truth given that the lie detector test has a positive result
The lie detector test gives a positive result 620 times
A person is telling the truth and the result gives a positive result 350 times.
Probability that a person is telling the truth given that the result is positive = (probability that a person is telling the truth and the machine gives a positive result)/(total number of positive results that the machine gives) = (350/620) = 0.5645
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