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July 22nd, 2009, 08:51 PM   #1
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probablity confusion

1) results from tests for marijuana use.

did the subject actually use marijuana
yes no
positive test result 119 24
negative test result 3 154

assume that 1 of the 300 test subject is randomly selected. find proof of getting someone who tests positive given that he/she did not use marijuana?
why is 24/300 wrong?



2)what is he probabilty that out of four children randomly chosen, exactly two are girls?

3)what is the probability that out of four children randomly chosen, exactly two are girls?

4) pedestrian intoxicated
yes no
yes 59 79
driver intoxicated no 266 581

if two different pedestrians deaths are randomly selected, find the prob that they both involved intoxicated drivers? (my teacher said the answer is 138/985 x 137/984, which really doesnt make sense to be, but if someone could explain it to me that would be great. thanks)
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July 23rd, 2009, 09:44 PM   #2
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Re: probablity confusion

What about questions 2 and 3

2)what is he probabilty that out of four children randomly chosen, exactly two are girls?
3)what is the probability that out of four children randomly chosen, exactly two are girls?
didn't you mean:
2)what is he probabilty that out of four children randomly chosen, exactly two are girls?
3)what is the probability that out of four children randomly chosen, at least two are girls?

could you please restate problem 4?
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July 23rd, 2009, 09:56 PM   #3
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Re: probablity confusion

Hello. I think I solved your problems but since I am new to theory of probability you should double check the logic.
The condition of the problem was the following :

_________________ yes | no
positive test result: 119 24|143
negative test result: 3 154|157
_________________ 122 178
Let A be the event that the selected person had a positive test result. And let B be the event that a selected person did not actually use marijuana.
We are looking for P[AB]. Since A and B are independent events , the probability of P[AB]=P[A]P[B]. Therefore the answer should be (143/300)*(178/300).
I assume getting the answer 24/300 was due to the following considerations. Let A be the same event, I already defined(P[A]=143/300), and let B be the event that the selected person had a positive test result,while he actually did not use marijuana( P[B]=24/143). Here B is clearly dependent of A so P[AB]!=P[A][B] ("!=" means not equal to).
According to my experience It is really important to define properly the probability space or the events you analyze. Keeping this into consideration good luck with the other questions.
(Actually I am not 100% sure that my answer is the correct one. So if I am mistaken , please excuse me)
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