Let A and B be two events of a random experiment. The probability that a second event (B) will occur if a first event (A) has already occured is called the conditional probability of B when A is given and is denoted by P(B/A).

• The conditional proabability is valid only when P(A) ≠ 0.

• Similarly, P(A/B) is the conditional probability of A when B is given.