Probability

Probability concerns events and numerical descriptions of how likely they are to occur. The probability of an event is a number between 0 and 1; the larger the probability, the more likely an event is to occur. This number is often expressed as a percentage (%), ranging from 0% to 100%.

A simple example is the tossing of a fair (unbiased) coin.
Since the coin is fair, the two outcomes are both equally probable; the probability of "heads" is the same as the probability of "tails".
Since no other outcomes are possible, the probability of either "heads" or "tails" is 1/2.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY-SA 3.0

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