Probability Created by Michele Hinkle and Jeffrey Fries

Probability Created by Michele Hinkle and Jeffrey Fries March 2005

Definitions Event: • Something that may or may not happen. Probability: • The chance of an event happening. Impossible: • An event that can never happen. Certain: • An event that must happen.

The Probability Continuum If an event has a probability of ½, some people say that there is a 50 -50 chance that the event will happen. 0 impossible unlikely 1/2 equally likely 1 certain

Making Predictions: Suppose you flip a coin 20 times. How many times will it land heads up? When you know the probability of an event, you can use this probability to make a prediction.

Since the probability of a coin landing heads up is 1/2, you can predict that if you flip a coin 20 times, it will probably land heads up ½ of 20, or 10 times. There is no guarantee that this will actually happen, but it is a reasonable prediction.

Example: Describe the likelihood of picking a green egg from the bag. • Since all the eggs are green, any egg you choose will be green. You are certain to pick a green egg. Number of favorable outcomes 4 Number of possible outcomes 4

• Since no eggs are green, whichever egg you choose will not be green. It is impossible to pick a green egg. Number of favorable outcomes Number of possible outcomes 0 4

• 2 of the 4 eggs, or ½ of the eggs are green. You have an equally likely chance of picking a green egg as not picking a green egg. Number of favorable outcomes Number of possible outcomes 2 4

• It is unlikely, but not impossible, that you will pick a green egg from the bag that has only one green egg. Number of favorable outcomes Number of possible outcomes 1 4

• It is likely, but not certain, that you will pick a green egg from the bag that has three green eggs. Number of favorable outcomes Number of possible outcomes 4 5

The Probability Continuum 0 impossible unlikely 1/2 equally likely 1 certain