1. Conditional Probability and Independence
5.3
Conditional Probability and Independence

2. READ: Who Has Pierced Ears? Pg 318

3. What is Conditional Probability?
The probability that one event happens given that another event is already known to have happened is called a conditional probability.
Suppose we know that event A has happened. Then the probability that event B happens given that event A has happened is denoted by P(B | A).
Read | as “given that” or “under the condition that”

4. Calculating Conditional Probabilities
To find the conditional probability P(A | B), use the formula
The conditional probability P(B | A) is given by
This is on the AP Formula Sheet

5. Who Reads the Paper?

6. LOOK AT: conditional distribution graph on pg 320

7. Do Check Your Understanding page 321 Calculating Conditional Probabilities
Consider the two-way table on page Define events
E: the grade comes from an EPS course, and
L: the grade is lower than a B.
Total
Total
6300
1600
2100
Find P(L)
Find P(E | L)
Find P(L | E)
Try Exercise 39
P(L) = 3656 / =
P(E | L) = 800 / 3656 =
P(L| E) = 800 / 1600 =

8. The General Multiplication Rule
The probability that events A and B both occur can be found using the general multiplication rule
P(A ∩ B) = P(A) • P(B | A)
where P(B | A) is the conditional probability that event B occurs given that event A has already occurred.
In words, this rule says that for both of two events to occur, first one must occur, and then given that the first event has occurred, the second must occur.
This is NOT on the AP Formula Sheet

9. Teens with Online Profiles – pg 322

10. Tree Diagrams and the General Multiplication Rule:
Look at Serve it Up! Pg 322

11. Who Visits YouTube?

12. Mammograms
A Randomly selected woman aged 40 or over from this region tests positive for breast cancer in a mammogram. Find the probability that she actually has breast cancer? Show your work.
 LOOK AT – table on page 326 and read summary!!

13. DO: Check Your Understanding – page 326

14. Conditional Probability and Independence
When knowledge that one event has happened does not change the likelihood that another event will happen, we say that the two events are independent.
Two events A and B are independent if the occurrence of one event does not change the probability that the other event will happen. In other words, events A and B are independent if
P(A | B) = P(A) and P(B | A) = P(B).
When events A and B are independent, we can simplify the general multiplication rule since P(B| A) = P(B).
Multiplication rule for independent events
If A and B are independent events, then the probability that A and B both occur is
P(A ∩ B) = P(A) • P(B)

15. Lefties Down Under

16. DO: Check Your Understanding – page 328

17. Multiplication Rule for Independent Events
On January 28, 1986, Space shuttle Challenger exploded on takeoff. All seven crew members were killed. Following the Space Shuttle Challenger disaster, it was determined that the failure of O-ring joints in the shuttle’s booster rockets was to blame. Under cold conditions, it was estimated that the probability that an individual O-ring joint would function properly was Assuming O-ring joints succeed or fail independently, what is the probability all six would function properly?
P( joint 1 OK and joint 2 OK and joint 3 OK and joint 4 OK and joint 5 OK and joint 6 OK)
By the multiplication rule for independent events, this probability is:
P(joint 1 OK) · P(joint 2 OK) · P (joint 3 OK) • … · P (joint 6 OK)
= (0.977)(0.977)(0.977)(0.977)(0.977)(0.977) = 0.87
There’s an 87% chance that the shuttle would launch safely under similar conditions (and a 13% chance that it wouldn’t).

18. Rapid HIV Testing

20. Read: Sudden Infant Death pg 331