# General Probability Rules

AP Statistics

6.3 General Probability Rules

General Probability Rules

Union: the event that AT LEAST ONE of the collection occurs (think OR)

Example: A single 6-sided die is rolled. What is the probability of rolling a 2 or a 5?

Possibilities: A two can be rolled, or a five can be rolled

These events are mutually exclusive since they cannot occur at the same time.

Example: In a math class of 30 students, 17 are boys and 13 are girls. On a unit test, 4 boys and 5 girls made an A grade. If a student is chosen at random from the class, what is the probability of choosing a girl or an A student?

Personal Probabilities

Probabilities or chance behavior based on ones personal opinion

Joint Events

Events that have a common outcome

Joint Probabilities: P(A and B)

Probabilities of joint events

Conditional Probability

• Written P(B|A) meaning “probability of B given A has occurred”
• Reduces the size of the Sample Space
• NOTE: it is easy to confuse probabilities and conditional probabilities of the same event

Example: A math teacher gave her class two tests. 25% of the class passed both tests and 42% of the class passed the first test. What percent of those who passed the first test also passed the second test?

Intersection

 two events occurring; all events are happening

(mainly two events A&B but not only in A or B alone)

More on Tree Diagrams

Make tree diagrams and assign probabilities to make problems easier

Example:In Orange County, 51% of the adults are males. (It doesn't take too much advanced mathematics to deduce that the other 49% are females.) One adult is randomly selected for a survey involving credit card usage.

a. Find the prior probability that the selected person is a male.

b. It is later learned that the selected survey subject was smoking a cigar. Also, 9.5% of males smoke cigars, whereas 1.7% of females smoke cigars (based on data from the Substance Abuse and Mental Health Services Administration). Use this additional information to find the probability that the selected subject is a male.

Let's use the following notation:

M = male M = female (or not male)

C = cigar smoker C = not a cigar smoker.

Solution:

a. Before using the information given in part b, we know only that 51% of the adults in Orange County are males, so the probability of randomly selecting an adult and getting a male is given by P(M) = 0.51.

b. Based on the additional given information, we have the following:

P(M) = 0.51 because 51% of the adults are males

P(M) = 0.49 because 49% of the adults are females (not males)

P(C|M) = 0.095 because 9.5% of the males smoke cigars (That is, the probability of getting someone who smokes cigars, given that the person is a male, is 0.095.)

P(C|M) = 0.017 because 1.7% of the females smoke cigars (That is, the probability of getting someone who smokes cigars, given that the person is a female, is 0.017.)

Let's now apply Bayes' theorem by using the preceding formula with M in place of A, and C in place of B. We get the following result:

Before we knew that the survey subject smoked a cigar, there is a 0.51 probability that the survey subject is male (because 51% of the adults in Orange County are males). However, after learning that the subject smoked a cigar, we revised the probability to 0.853. There is a 0.853 probability that the cigar−smoking respondent is a male. This makes sense, because the likelihood of a male increases dramatically with the additional information that the subject smokes cigars (because so many more males smoke cigars than females).

HW: pg. 440; 6.70, 6.71, 6.72 6.73, 6.82, 6.90