Probability is a quantitative statement of the likelihood or chance that some specified event has happened or will happen in the future. Probability is a number between zero and one, and is often abbreviated with the letter "p". Zero probability means that the event cannot happen (i.e., it is impossible). A probability of one means that an event is sure to happen (i.e., a certainty). Probability can be expressed as a fraction, where the probability p=0.01 is the same as saying p = 1/100.

The concepts of probability are perhaps best understood by way of examples. Common examples use dice, coins, or cards to illustrate probability in concrete ways. For example, what is the chance of rolling a four with a single roll of one die? If the die is conventional, with one number on each of its six faces, and the die is...