Exercise from lecture: The CIA and FBI use polygraph machines (lie detectors) to

Question

Exercise from lecture: The CIA and FBI use polygraph machines (lie detectors) to screen job applicants. A review by a federal panel of scientists reported that "if polygraphs were administered to a group of 10000 people that included ten spies, 1600 innocent people would fail the test and two spies would pass." [Source: Washington Post, May 1, 2006]. A person is selected at random, what is the probability that the person fails the test? Suppose a person fails the test, what is the probability that the person is a spy?

Explanation / Answer

In the population of 10,000 including 10 spies, if the test is done 1600 innocent people fail the test and 2 spies pass. this means that 8 spies fail the test. therefore total people who fail = 1608

a) A person selected at random fails the test: This means that the person could either be innocent or a spy.

P= Favourable outcomes/total outcomes

P= 1608C1/10000C1= 1608/10000= 0.01608

b) if a person fails the test, the probability that he is a spy

This means that out of 1608, what is the probabilty that a person who failed is a spy.

P= Favourable outcomes/total outcomes

P= 8C1/1608C1= 8/1608= 0.005

Related Questions

Navigate

• Browse (All)
• Browse E
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.