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March 10th, 2011, 03:59 PM  #1 
Newbie Joined: Feb 2009 Posts: 21 Thanks: 0  bayes theorem question
You are serious student who studies on Friday nights but your roommate goes out and has a good time. 40% of the time he goes out with his girlfriend; 60% of the time he goes to a bar. 30% of the times when he goes out with his girlfriend he spends the night at her apartment. 40% of the times when he goes to a bar he gets in a fight and gets thrown in jail. You wake up on saturday morning and your roommate is not home. What is the probability that he is in jail? Attempt : A = Not Home B = In Jail p(BA) = P(B n A )/ P(A) P(A) = 0.4*0.3 + 0.6 * 0.4 =0.36 P(B n A) = (0.6)(0.4) = 0.24 Thus P(BA) = 0.24 / 0.36 = 2/3 Is that correct? 
March 11th, 2011, 08:30 AM  #2 
Math Team Joined: Dec 2006 From: Lexington, MA Posts: 3,267 Thanks: 407  Re: bayes theorem question Hello, tnutty! Absolutely correct! . . . Good work! 
March 17th, 2011, 02:52 AM  #3 
Newbie Joined: Mar 2011 Posts: 1 Thanks: 0  Re: bayes theorem question
Hi, I need to calculate certain probability but there is also multivariate probability that I am using. Is this formula correct? P(something) = P(ST) * P(IP?ST) * P(OP?(IP?ST)) = P(ST) * P(IP?ST) * (P(ST?IP?OP))/(P(IP?ST)) = P(ST) * P(IP?ST) * (P(ST?IP?OP))/(P(IP?ST)*P(ST)) = P(ST?IP?OP) 

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