In any random experiment, a chance, or probability, always exists that a particular event will or will not occur. The probability that event A will occur is the ratio of the number of outcomes favorable to A to the total number of possible outcomes.

You can assign a number between zero and one to an event as an indication of the probability that the event will occur. If you are absolutely sure that the event will occur, its probability is 100% or one. If you are sure that the event will not occur, its probability is zero.