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November 17th, 2012, 08:11 PM  #1 
Newbie Joined: Nov 2012 Posts: 1 Thanks: 0  probability set theory  probability of x out of n events
am having an issue related to probability set theory with intersection/union terms. When calculating the union of terms or in other words, the probability that at least one term "fails", it can be written as the following for three terms: P(A+B+C) = P(A)+P(B)+P(C)P(AB)P(AC)P(BC)+P(ABC). My question is how to assess a problem similar to this when we are looking at the probability at least x terms out of n terms fail. For example, at least 2 out of 4. Or at least 7 out of 10. I thought I came up with the correct answer when looking at a system of only 4 terms. For example: P(at least 2 out of 4) = P(AB)+P(AC)+P(AD)+P(BC)+P(BD)+P(CD) 2*(P(ABC)+P(ABD)+P(ACD)+P(BCD)) +3*(P(ABCD)) P(at least 3 out of 4) = P(ABC)+P(ABD)+P(ACD)+P(BCD) 3*(P(ABCD)) This works here. In fact, it works for P(at least 2 out of n) and P(at least (n1) out of n) for all cases of n. However it does not work for the situations in between. I am looking for an analog formula that can evaluate any case for the probability of at least x out of n failure. Any help with be appreciate. Thanks! 

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events, probability, set, theory 
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