Chapter 4 Probability Definitions A probability experiment is
Chapter 4 Probability
Definitions • A probability experiment is a chance process that leads to well-defined results called outcomes. • An outcome is the result of a single trial of a probability experiment. • A sample space is the set of all possible outcomes of a probability experiment.
The probability of any event E is
• The probability of an event is a number (either a fraction or a decimal) between zero and one, inclusive.
Cardinal Rule • Never Count Anything Twice!!!
• If the probability of an event is zero, the event is impossible. • If the probability of an event is one, the event is certain. • The sum of the probabilities of the outcomes in the sample space is one.
Cardinal Rule • Never Count Anything Twice!!!
Example 1 • Roll a fair die. – Find the sample space.
Example 2 • A family wants to have three children. – How many elements are in the sample space? – List the elements in the sample space.
Example 1 • Roll a fair die. – Find the sample space. – Find the probability of rolling an even number. – Find the probability of rolling a number that is prime.
Example 2 • A family wants to have three children. – How many elements are in the sample space? – List the elements in the sample space. – Find the probability that the family has one girl. – Find the probability that the family has at most one girl. – Find the probability that the family has at least one boy.
Example 3 • A card is drawn from an ordinary deck of cards. – Find the probability of getting a jack. – Find the probability of getting the 6 of clubs. – Find the probability of getting a red card. – Find the probability of getting a red jack. – Find the probability of getting a red card or a jack. – Find the probability of getting a jack and a king.
Cardinal Rule • Never Count Anything Twice!!!
• Two events are mutually exclusive if they cannot occur at the same time.
Example 4 • A jar contains 3 red, 2 blue, 4 green, and 1 white marble. A marble is chosen randomly. – Find the probability of choosing a red marble. – Find the probability of choosing a red or a blue marble. – Find the probability of choosing a red and a blue marble.
Example 5 • A jar contains 3 red, 2 blue, 4 green, and 1 white marble. Two marbles are randomly chosen, choosing the 2 nd marble after replacing the first. – Find the probability of choosing 2 red marbles. – Find the probability of choosing a red and then a green marble. – Find the probability of choosing a red and a green marble. – Find the probability of choosing two white marbles.
• Two events A and B are independent if the fact that A occurs does not affect the probability of B occurring.
Example 6 • A jar contains 3 red, 2 blue, 4 green, and 1 white marble. Two marbles are randomly chosen, choosing the 2 nd marble without replacing the first. – Find the probability of choosing 2 red marbles. – Find the probability of choosing a red marble and then a green marble. – Find the probability of choosing two white marbles.
• When the outcome or occurrence of the first events affects the outcome or occurrence of the second event in such a way that the probability is changes, the events are said to be dependent events.
Example 7 • If 25% of U. S. federal prison inmates are not U. S. citizens, find the probability that two randomly selected federal prison inmates will not be U. S. citizens.
Example 8 • A flashlight has 6 batteries, two of which are defective. If two batteries are randomly selected, find the probability that both are defective.
Example 9 • In a large shopping mall, a marketing agency conducted a survey on credit cards. The results are below.
Employment Owns a Status credit card Employed 18 Unemployed 28 Does not own a credit card 29 34
Cardinal Rule • Never Count Anything Twice!!!
• If one person is randomly selected, – Find the probability that the person owns a credit card. – Find the probability that the person is employed. – Find the probability that the person owns a credit card and is employed. – Find the probability that the person owns a credit card or is employed. – Find the probability that the person owns a credit card given that the person is employed. – Find the probability that the person is employed given that the person owns a credit card.
• If you see the word “given” in a problem, the sample space has changed. • The new sample space is what comes after the word “given” • In other words, in our formula for probability, the denominator changes to what comes after the word “given”
Cardinal Rule • Never Count Anything Twice!!!
• Are the events “owning a credit card” and “being employed” mutually exclusive events? Why or why not?
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