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## Probability

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**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.**unlikely**likely 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 1/2 equally likely 1 certain**Making Predictions: Suppose you flip a coin20 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.**Number of favorable outcomes**Number of possible outcomes 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. 4 4**Number of favorable outcomes**Number of possible outcomes • Since no eggs are green, whichever egg you choose will not be green. It is impossible to pick a green egg. 0 4**Number of favorable outcomes**Number of possible outcomes • 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. 2 4**Number of favorable outcomes**Number of possible outcomes • It is unlikely, but not impossible, that you will pick a green egg from the bag that has only one green egg. 1 4**Number of favorable outcomes**Number of possible outcomes • It is likely, but not certain, that you will pick a green egg from the bag that has three green eggs. 4 5**unlikely**likely The Probability Continuum 0 impossible 1/2 equally likely 1 certain