This is the complement rule. It is used to determine the probability of an event occurring by subtracting the probability of the event not occurring from 1. This rule is useful because sometimes it is easier to calculate the probability of an event happening by determining the probability of it not happening and subtracting the result from 1. Notice that the events A and ˜A are mutually exclusive and collectively exhaustive. Therefore, the probabilities of A and ˜A sum to 1. A Venn diagram illustrating the complement rule is shown as: