Some Probability Formulas

Some useful probability formulas

The Addition Rule

Consider 2 events, A and B. To find the probability that either A or B (or both) will occur, we can use the following formula, called the addition rule:

We subtract the probability of both A and B occurring, as to not "double count" them. This can seen better by looking at the following Venn Diagram:

Conditional Probability

Sometimes we would like to know the probability of an event B occurring, given that another event A has already occurred. This in called conditional probability and we use the notation:

This is read as "The probability of B given A," and can be calculated as follows:

Contributed by David Ciskowski