Suppose we are interested in two events, *A* and *B*. In this case, event *A* might represent the event that a patient has appendicitis and event *B* might represent a patient having a high white blood cell count. The **conditional probability** of event *A* given event *B* is essentially the probability that event *A* will occur when we know that event *B* has already happened.

Formally, we define the conditional probability of event *A* given event *B* as the joint probability of both events occurring divided by the probability of event *B* occurring:

Note that this is consistent with the way in which we define statistical independence. Statistical independence ...

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