The conditional probability of an event A, assuming that an event B has occurred, is defined by P(A|B) = P(A∩B) / P(B). if P(B) gt; 0, A, B ⊂ S. The probability P(A|B) is usually different from the probability P(A) because knowledge, that an event B has occurred, has an influence on the estimate of the probability P(B). Example: We are closing eyes and rolling the die. What is the probability of getting an even number? P(A) = 3/6 = 1/2. What is the probability of getting an even number (the event A), assuming (somebody has told us) that number on the die is less than 6 (the event B)? P(A|B) = P(A∩B) / P(B) = 2/5.