What is Conditional Probability?
Two events A and B are said to be dependent when B can occur only when A is known to have occurred (or vice versa). The probability attached to such an event is called the conditional probability and is denoted byP(A/B) or, in other words probability of A given that B has occurred.If two events A and B are dependent, then the conditional probability of B given A is:P(B/A) = P(AB)/P(A)Proof: suppose a_{1} is the number of cases for the simultaneous happening of A and B out of a_{1} + a_{2} cases in which A can happen with or without happening of B.∴ P(B/A) = a_{1}/(a_{1} + a_{2}) = (a_{1}/n)/(a_{1} + a_{2})/n = P(AB)/P(A)Similarly it can be shown thatP(A/B) = P[(AB)/P(B)]The general rule of multiplication in its modified form in terms of conditional probability becomes:P(A and B) = P(B) × P(A/B)Or, P(A and B) = P(A) × P(B/A)For three events A, B and C we haveP(ABC) = P(A) × P(B/A) × P(C/AB)i.e. the probability of occurrence of A, B and C is equal to the probability of A, times of the probability of B given that A has occurred, times the probability of C given that both A and B have occurred.Illustration: a bag contains 5 white and 3 black balls. Two balls are drawn at random one after the other without replacement. Find the probability that both balls drawn are black.Solution: probability of drawing a black ball in the first attempt isP(A) = 3/(5 + 3) = 3/8Probability of drawing the second black ball given that the first ball drawn is blackP(B/A) = 2/(5 + 2) = 2/7∴ The probability that both balls drawn are black is given byP(AB) = P(A) × P(B/A) = 3/8 × 2/7 = 3/28
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