Probability is the branch of mathematics that deals with the chances of an event occurring. Consider the possible outcomes of tossing two quarters—head-head, head-tail, tail-head, or tail-tail. Note that two out of four outcomes have one head and one tail. In probability, each collection of outcomes is called an event, and those that cannot be broken into simpler components are called simple events. The probability of an event is given by the number of ways it can occur divided by the total number of different simple events. It can be calculated for each case. For any event, its probability can range between 0 and 1. For an impossible event, it is 0, and for a certain event, it is 1. Probability is highly useful in statistics. Using laws of probability, statisticians can draw inferences from past events and predict future outcomes. For instance, the computed probabilities of the coin toss experiment can be used to construct a probability distribution. Comparing actual outcomes with these theoretical probabilities will determine if the outcomes are unusual.