01:960:285 Lecture Notes - Lecture 12: Joint Probability Distribution, Conditional Probability, Sample Space
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Recall that the probability of an event occurring given that another event has already occurred is called a conditional probability. The probability that event b occurs, given that event a has already occurred is. This formula comes from the general multiplication principle and a little bit of algebra. Since we are given that event a has occurred, we have a reduced sample space. Instead of the entire sample space s, we now have a sample space of a since we know. So the old rule about being the number in the event divided by the number in the sample space still applies. It is the number in a and b (must be in a since a has occurred) divided by the number in a. If you then divided numerator and denominator of the right hand side by the number in the sample space s, then you have the probability of a and b divided by the probability of a.