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STA 301 – Applied Statistics
Spring 2008, Section B
Homework #5 
Key
Due Friday, February 29
Question 1
:
(p. 73, #2.109)
A truth serum has the property that 90% of the guilty suspects are properly judged while,
of course, 10% of guilty suspects are improperly found innocent.
On the other hand,
innocent suspects are misjudged 1% of the time.
If a suspect is selected from a group of
people of whom only 5% have ever committed a crime, and the serum indicates that he is
guilty, what is the probability that he is innocent?
Define G = “guilty” and T = “tests guilty”.
Then from the description of the problem,
we have:
Using Bayes’ Rule, we obtain
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 Spring '08
 Noe
 Statistics

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