Probability

A number between 0 and 1 which represents how likely an event is to occur. Events with probability equal to 0 never occur. Events with probability equal to 1 always occur.

In data analysis, probability is normally defined in terms of the relative frequency of occurrence of an event which can be repeated many times. For example, if you repeatedly sample temperatures from a process and get values below 150 degrees half the time, then the "probability" of getting a reading below 150 degrees is equal to 0.5 or 50%.

In daily life, we sometimes use probability in a different sense, i.e., to express our degree of belief about the likelihood of an event which can not be repeated indefinitely under identical conditions. For example, you might say that the chance of getting a raise this year is "one in a million". Such "subjective" probabilities are sometimes used in statistical decision theory.

See also: Bayes Rule, Cumulative Probability, Permutations and Combinations, Statistic.