 # Multiplication Rule: Basics

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Multiplication Rule: Basics
Section 4-4 Multiplication Rule: Basics

NOTATION P(A and B) = P(event A occurs in a first trial and event B occurs in a second trial)

EXAMPLES Suppose that you first toss a coin and then roll a die. What is the probability of obtaining a “Head” and then a “2”? A bag contains 2 red and 6 blue marbles. Two marbles are randomly selected from the bag, one after the other, without replacement. What is the probability of obtaining a red marble first and then a blue marble?

CONDITIONAL PROBABILITY
If event B takes place after it is assumed that event A has taken place, we notate this by B|A. This is read “B, given A.” P(B|A) represents the probability of event B occurring after it is assumed that event A has already occurred.

INDEPENDENT AND DEPENDENT EVENTS
Two events A and B are independent if the occurrence of one event does not affect the probability of the occurrence of the other. Several events are independent if the occurrence of any does not affect the occurrence of the others. If A and B are not independent, they are said to be dependent.

FORMAL MULTIPLICATION RULE
P(A and B) = P(A) · P(B|A) NOTE: If events A and B are independent, then P(B|A) = P(B) and the multiplication rule simplifies to P(A and B) = P(A) · P(B)

APPLYING THE MULTIPLICATION RULE

INTUITIVE MULTIPLICATION RULE
When finding the probability that event A occurs in one trial and B occurs in the next trial, multiply the probability of event A by the probability of event B, but be sure that the probability of event B takes into account the previous occurrence of event A.

EXAMPLES What is the probability of drawing an “ace” from a standard deck of cards and then rolling a “7” on a pair of dice? In the 105th Congress, the Senate consisted of 9 women and 91 men, If a lobbyist for the tobacco industry randomly selected two different Senators, what is the probability that they were both men? Repeat Example 2 except that three Senators are randomly selected.

EXAMPLE In a survey of 10,000 African-Americans, it was determined that 27 had sickle cell anemia. Suppose we randomly select one of the 10,000 African-Americans surveyed. What is the probability that he or she will have sickle cell anemia? If two individuals from the group are randomly selected, what is the probability that both have sickle cell anemia? Compute the probability of randomly selecting two individuals from the group who have sickle cell anemia, assuming independence.

TREATING DEPENDENT EVENTS AS INDEPENDENT: THE 5% GUILDELINE FOR CUMPBERSOME CALCULATIONS
If a sample size is no more than 5% of the size of the population, treat the selections as being independent (even if the selections are made without replacement, so they are technically dependent).