A crime has been committed by a solitary individual, who left some DNA at the scene of the crime. Forensic scientists who studied the recovered DNA noted that only five strands could be identified and that each innocent person, independently, would have a probability of $10^{-5}$ of having his or her DNA match on all five strands. The district attorney supposes that the perpetrator of the crime could be any of the $1000,000$ residents of the town. $10,000$ of these residents have been released from prison within the past $10$ years; consequently, a sample of their DNA is on file. Before any checking of the DNA file, the district attorney feels that each of the ten thousand ex-criminals has probability $\alpha$ of being guilty of the new crime, while each of the remaining $990,000$ residents has probability $\beta$, where $\alpha$ = $c\beta$. (That is, the district attorney supposes that each recently released convict is $c$ times as likely to be the crime’s perpetrator as is each town member who is not a recently released convict.) When the DNA that is analyzed is compared against the database of the $10,000$ ex-convicts, it turns out that A. J. Jones is the only one whose DNA matches the profile. Assuming that the district attorney’s estimate of the relationship between α and β is accurate, what is the probability that A. J. is guilty?

The above is a question from First Course in Probability by Sheldon Ross.The solved question is at page 93(of the pdf) in the linked pdf.

Now, the solution calculates $P$(all in database innocent) as $=$ $1 -10,000\alpha$ .

My Understanding : $P$(all in database innocent) = $1-$ $P$(at least one guilty in the database), but since only one can be guilty, all events ($i^{th}$ ex-criminal being guilty) are mutually exclusive, hence $P$(at least one guilty)= $\alpha+\alpha+..+\alpha$= $10000\alpha$.

Doubt : Why is $P$(all in database innocent) $\ne$ $(1 -\alpha)^{10,000}$? or Is $(1 -\alpha)^{10,000}$ $=$ $1 -10,000\alpha$?.

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1 Answers1


Your reasoning in the "My understanding" paragraph is right.

Why would it be $(1-\alpha)^{10000}?$ You multiply probabilities for independent events, but the events "ex-convict A is guilty" and "ex-convict B is guilty" are not independent. On the contrary, you have correctly identified them as being mutually exclusive, which is about as far from independent as you can come.

hmakholm left over Monica
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  • Well, it's embarrassing to say the least. For anyone else, this may help : http://math.stackexchange.com/a/941158/93949 – PleaseHelp Aug 27 '15 at 15:18
  • In the above answer there is a comment - "If two events are mutually exclusive, then they are NOT independent" . Do you agree ? - I think the statement fails when any one of the events has zero probability. – PleaseHelp Aug 27 '15 at 15:22
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    @AtulGangwar: There is one loophole: If one of the events has probability $0$, then they can be both mutually exclusive and independent. However two events _with positive probability_ cannot be at once mutually exclusive and independent. – hmakholm left over Monica Aug 27 '15 at 15:25
  • I think it is a criminal act to include a link to an illegal copy of a textbook. The editors of this forum better remove the link to the illegal copy of the Ross book – user164118 Aug 27 '15 at 17:35