Statistics Dictionary

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Addition Rule

The rule of addition applies to the following situation. We have two events from the same sample space, and we want to know the probability that either event occurs.

Rule of Addition. If events A and B come from the same sample space, the probability that event A and/or event B occur is equal to the probability that event A occurs plus the probability that event B occurs minus the probability that both events A and B occur.

P(A B) = P(A) + P(B) - P(A ∩ B)

Note: Invoking the fact that P( A ∩ B ) = P( A )P( B | A ), the Addition Rule can also be expressed as:

P(A B) = P(A) + P(B) - P(A)P( B | A )

See also:   Rules of Probability | Probability Calculator