Statistics & Probability
Intermediate

Conditional Probability

Probability of A given B has occurred.

Formula

P(A∣B)=P(A∩B)P(B)P(A|B) = \frac{P(A \cap B)}{P(B)}

Variables

A, BEvents

Example

Drawing 2 aces from a deck: P(2nd ace | 1st ace) = 3/51 ≈ 0.059.

Did You Know?

Conditional probability is why "rare disease, positive test" often still means you're probably fine.

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