In this chapter we studied probability. We began by defining an event to be an experimental outcome that may or may not occur and by defining the probability of an event to be a number that measures the likelihood that the event will occur. We learned that a probability is often interpreted as a long-run relative frequency, and we saw that probabilities can be found by examining sample spaces and by using probability rules. We learned several important probability rules-addition rules, multiplication rules, and the rule of complements. We also studied a special kind of probability called a conditional probability, which is the probability that one event will occur given that another event occurs, and we used probabilities to define independent events.