Probability is how likely something is to happen. Whenever we're unsure about the outcome of an event, we can talk about the probabilities of certain outcomes - how likely they are. The analysis of events governed by probability is called statistics. The probabilities in a probability model must sum to 1. When the outcomes of an experiment are all equally likely, we can find the probability of an event by dividing the number of outcomes in the event by the total number of outcomes in the sample space for the experiment. Probability = the number of ways of achieving success. The total number of possible outcomes. For example, the probability of flipping a coin and it being heads is ½, because there is 1 way of getting a head and the total number of possible outcomes is 2 (a head or tail). We write P (heads) = ½. Chapter 22 explains the concept of the experimental approach, empirical probability and complimentary events.