Quantitative Method

Suppose A and B are events.
The probability that event B occurs given that event A occurred is called a conditional probability。For example:
When a balanced die is rolled, six equally likely outcomes are possible, let F=event a 5 is rolled, O=event the die comes up odd.
Under these circumstances, event O (odd) can occur in two ways, if a 1 or a 3 is rolled. Thus the conditional probability that the die comes up odd given that a 5 is not rolled equals
P(O/(not F)) = f/N =2/5 = 0.40
Conditional probability plays a central role in the analysis of cross-classified data.