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Rules of probability
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Rule 1:

number of results giving the event

The probability of an event = 

total number of possible results


It is a number between 0 and 1 inclusive.


Rule 2:

The probability P(A+B) that at least one of the events A and B occurs is given by:

P(A+B) < or = P(A) + P(B)

where P(A) and P(B) are the probabilities of A and B respectively. The equality applies if events A and B are exclusive, i.e. the occurrence of one precludes the other.


Rule 3:

The probability P(AB) of obtaining both A and B is given by:

P(AB) = P(A/B)P(B) [or P(B/A)P(A)]

where P(A/B) is the probability of A occurring, given that B has occurred already.

If A and B are independent, P(AB) = P(A)P(B).



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1999, 2000 The University of Liverpool, Department of Physics

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