A random experiment is any process that leads to two or more possible outcomes, without knowing exactly which outcome will occur.
For example, when a fair die is rolled we know that one of the six faces will show up but we will not be able to say exactly which face will actually show up. Thus, rolling a fair die is an example of a random experiment. Some further examples of random experiments are:
• Customers arriving at a particular store during some time interval.
• Airplanes taking off in a given time interval at some airport.
• Some particular customer requests in a bank.
Note that in a random experiment, although we do not know which outcome will occur, we are able to list or describe all of the possible outcomes.The set of all possible outcomes is important to understand the random experiment. Terefore, it is called a sample space, which is restated below as a definition.