Laws of Probability

Following are four basic laws of probability:

Law 1 : If the probability of an event is 1, then the event must occur.
For example, the probability of each of us dying is 1. We know that dying is certain to occur.

Law 2: If the probability of an event is 0, then the event will never occur.
For example, the probability of a person who was born outside the United States becoming
its president is zero. This is the decree of the U.S. Constitution.

Law 3: The probability of any event must assume a value between 0 and 1, inclusively.
For example, the probability of its raining today is 0.7 = 70 percent. We cannot be more
than 100 percent certain that it will rain, nor we cannot be less than 0 percent certain that it
will rain.

Law 4: The sum of the probabilities of all the simple events in a sample space must be equal
to 1. Another way of saying this is to say that the probability of the sample space in any
experiment is always 1.

For example, if we consider the sample space for Example 7-4, there are 8 simple events.
By the classical approach, each simple event has an equal chance of occurring. That is,
1 A each simple event has a - chance of occurring. When we sum these probabilities, we have 8*1/8 = 1