# Question: In a murder trial in Los Angeles the prosecution claims

In a murder trial in Los Angeles, the prosecution claims that the defendant was cut on the left middle finger at the murder scene, but the defendant claims the cut occurred in Chicago, the day after the murders had been committed. Because the defendant is a sports celebrity, many people noticed him before he reached Chicago. Twenty- two people saw him casually, one person on the plane to Chicago carefully studied his hands looking for a championship ring, and another person stood with him as he signed autographs and drove him from the airport to the hotel. None of these 24 people saw a cut on the defendant’s finger. If in fact he was not cut at all, it would be extremely unlikely that he left blood at the murder scene.

a. Because a person casually meeting the defendant would not be looking for a cut, assume that the probability is .9 that such a person would not have seen the cut, even if it was there. Furthermore, assume that the person who carefully looked at the defendant’s hands had a .5 probability of not seeing the cut even if it was there and that the person who drove the defendant from the airport to the hotel had a .6 probability of not seeing the cut even if it was there. Given these assumptions, and also assuming that all 24 people looked at the defendant independently of each other, what is the probability that none of the 24 people would have seen the cut, even if it was there?

b. What is the probability that at least one of the 24 people would have seen the cut if it was there?

c. Given the result of part b and given the fact that none of the 24 people saw a cut, do you think the defendant had a cut on his hand before he reached Chicago?

d. How might we estimate what the assumed probabilities in part a would actually be?

a. Because a person casually meeting the defendant would not be looking for a cut, assume that the probability is .9 that such a person would not have seen the cut, even if it was there. Furthermore, assume that the person who carefully looked at the defendant’s hands had a .5 probability of not seeing the cut even if it was there and that the person who drove the defendant from the airport to the hotel had a .6 probability of not seeing the cut even if it was there. Given these assumptions, and also assuming that all 24 people looked at the defendant independently of each other, what is the probability that none of the 24 people would have seen the cut, even if it was there?

b. What is the probability that at least one of the 24 people would have seen the cut if it was there?

c. Given the result of part b and given the fact that none of the 24 people saw a cut, do you think the defendant had a cut on his hand before he reached Chicago?

d. How might we estimate what the assumed probabilities in part a would actually be?

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