Probability

Created byMichele Hinkle and Jeffrey Fries

March 2005

DefinitionsEvent:• 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

0

impossible

1

certain

1/2

equally

likely

If an event has a probability of ½, some

people say that there is a 50-50 chance that the

event will happen.

unlikely likely

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.

4Number of favorable outcomes

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

0

Number of possible outcomes

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

2

Number of possible outcomes

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

1

Number of possible outcomes

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

4

Number of possible outcomes

5

The Probability Continuum

0

impossible

1

certain

1/2

equally

likely

unlikely likely