In: Statistics and Probability
A truth serum given to a suspect is known to be 82 percent
reliable when the person is guilty and 94 percent reliable when the
person is innocent. In other words, 18 percent of the guilty are
judged innocent by the serum and 6 percent of the innocent are
judged guilty. If the suspect was selected from a group of suspects
of which only 8 percent are guilty of having committed a crime, and
the serum indicates that the suspect is guilty of having committed
a crime, what is the probability that the suspect is innocent?
(Round your answer to 3 decimal places.)
Probability:
(0.841 is incorrect, taken from a different post of this
question)
P(suspect is guilty) = 0.08
P(judged innocent | guilty) = 0.18
P(judged guilty | guilty) = 1 -0.18 = 0.82
P(judged guilty | innocent) = 0.06
P(judged guilty) = P(judged guilty | guilty) * P(guilty) + P(judged guilty | innocent) * P(innocent)
= 0.82 * 0.08 + 0.06 * (1 - 0.08)
= 0.1208
P(innocent | judged guilty) = P(judged guilty | innocent) * P(innocent) / P(judged guilty)
= 0.06 * (1 - 0.08) / 0.1208
= 0.457 (ans)