General multiplication rule

The general multiplication rule says P(A and B) equals P(A) times the conditional probability of B given A, and it holds for any two events.

The rule is P(AB)=P(A)P(BA)P(A \cap B) = P(A) \cdot P(B \mid A), where \cap is the intersection symbol (A and B) and P(BA)P(B \mid A) is read the probability of B given A. No independence is required, because the conditional factor already accounts for any way the first event shifts the second. Drawing two cards from a standard 52-card deck without replacement, P(two aces)=452351=12210.0045P(\text{two aces}) = \frac{4}{52} \cdot \frac{3}{51} = \frac{1}{221} \approx 0.0045. When the events happen to be independent, P(BA)=P(B)P(B \mid A) = P(B) and the rule collapses to the simple product.

Where this comes up

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