Probability is a number that expresses the likelihood or chance that something is or will be true. Probability is ALWAYS a number between zero and one, although sometimes it is expressed as a percentage, in which case we should more accurately called it "percent chance" and not probability.
A probability of zero means there is no chance the event in question is true or will happen. A probability of one means it's a sure thing. Intermediate probabilities express intermediate levels of confidence in the event. For example, a probability of .5 expresses a 50-50 chance--just as likely to happen as not happen.
Although we often calculate probabilities of past events, it is best to think of a probability as a prediction of the future. This will prevent you from making certain kinds of logical errors about probabilities.
A concept related to probability is odds. We will discuss odds in class when it becomes necessary. (And it will.)
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