• The conditional probability of A given B is the probability that A occurs, knowing that B has already occurred.
• It is written P(A | B).
• The idea is that the sample space shrinks to the event B, and we look for the outcomes of A inside B only.
• It is calculated by:

P(A | B) = P(A ∩ B) ÷ P(B), where P(B) ≠ 0

1. It can also be found from a two-way table by taking only the row or column of B
2. and
3. finding the fraction of that row or column that satisfies A.