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Probability: The chance or likelihood that an event will occur.  It is usually expressed as a fraction, but can be given as a decimal or percent.

A Probability of 0 means that the event can never occur (for example rolling a 0 on a six-sided dice numbered 1-6)

A Probability of 1 means that the event will always occur (for example rolling a number on a six-sided dice)


Experimental Probability: The probability of an event based on the results of an experiment.

For example: If I wanted to know what the probability is of someone choosing Coke or Diet Coke, I can design an experiment and survey 200 people.  Lets say 120 chose Diet Coke and 80 chose regular coke.  The experimental probability of choosing Diet coke is .

Theoretical Probability: The probability of an event, P(E) is the number of favorable outcomes divided by the number of total outcomes.

For example: in the above problem the theoretical probability would be because you have a 1 in two chance of choosing Diet Coke (Diet coke would be the favorable outcome and there are two total possibilites)