Simple Probability

The probability of a simple event is the ratio of the number of favorable outcomes for the event to the total possible outcomes of the event.

The list of all possible outcomes is called the sample space.

For example, if you ask a friend to go the movies, and you tell your friend that you are available for either the 2:00 pm show or the 5:00 pm show, then the probability of your friend saying 'yes' are 2/3 or about 67%.

The sample space for this example is 3; you friend can say:

1.Yes to the 2:00 pm show

2. Yes to the 5:00 pm show

3. No to either show.

The number of favorable outcomes is 2 (the two 'yes' answers).

The odds of an event occurring is the ratio of the number of favorable outcomes to the number of unfavorable outcomes.

In the example above, the odds of your friend's accepting your invitation are 2:1, or two possible 'yes' answers to one possible 'no' answer.

© The e-Learning Research Foundation 2007