# Basic Probability Theory II

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1 RECAP Basic Probability heory II Dr. om Ilvento FREC 408 We said the approach to establishing probabilities for events is to Define the experiment List the sample points Assign probabilities to the sample points Determine the collection of sample points contained in the event of interest Sum the sample point probabilities to get the event probability Next Complementary Events Compound Events Union of Events Intersection of Events General Additive Rule Additive Rule for Mutually Exclusive Events Multiplicative Law Conditional Probability and Independent Events Equally Likely Mutually Exclusive Events Probability Rules in Book Rule 1. Additive rule for mutually exclusive events. If two events are mutually exclusive, then the probability that either Event A or Event B occurs is equal to the sum of their probabilities. A or = Ac = A) + (p 200) Rule 2. he Probability for an experiment in one of a number of equally likely mutually exclusive events. If an experiment can result in only one of M equally likely events, and that m of these events result in event A, then the probability of Event A is: A) = m/m (p 201) Probability Rules in Book Rule 3. Probability Relationship for Complementary Events A) = 1- A c ) (p203) Rule 4. he Probability that Both of wo Independent Events A and B occur. If two events A and B are independent, then the probability that both A and B occur is equal to the product of their respective unconditional probabilities: A and =A =A)* (p214) Probability Rules in Book Rule 5. he Multiplicative Law of Probability. he probability that both of two events A and B occur is A and =A = A)*B A) = *A (p221) Rule 6. he Additive Law of Probability. he probability that either event A or event B or both occur is: A or = P (A = A)+- A (p222) 1

2 Complementary Events he complement of an event A is the event that A does not occur that is all sample points not in Event A Denoted as A C or as A in the book A) + A c ) = 1.0 Probability of Complementary Events: A c ) = 1- A) (RULE #3 p203) Compound Events Events can be comprised of several events joined together, and these are called COMPOUND EVENS hey can be the UNION of several events Or the INERSECION of several events Union of two Events he union of two events, A and B is the Event that occurs if either A, B, or both occur on a single performance of the experiment (Def4.11 P221) We denote the Union as A c B A c B consists of all the sample points that belong to A or B or both. Intersection of two Events he intersection of two events, A and B is the Event that occurs if both A and B occur on a single performance of the experiment (Def4.10 P220) We denote the Intersection as A 1 B A 1 B consists of all the sample points that belong to both A and B. Example using a die toss Event A [oss an even number] Event B [oss a number <= 3] What is A c? What is A c B? What is A 1 B? Can you calculate the probability of the complement, union and the intersection of these events? What is A c for a roll of a die? A = [ 2, 4, 6] A c = [1, 3, 5] the event A c is tossing an odd number he probability of this event is 3/6 or ½ A c ) = 1) + 3) + 5) = 3/6 = ½ Alternative approach: A c ) = 1- A) =1- {2) + 4) + 6)} = 1-3/6 = 1-1/2 = ½ employing Rule#3 2

3 What is A c B for a roll of a die? What is A c B for a roll of a die? A = [ 2, 4, 6] B = [1, 2, 3] A c B = [2, 4, 6] + [1, 2, 3] = [1, 2, 3, 4, 6] Note that the sample point 2 is common to both events and we don t count it twice. 4 A B 5 S What is A c for a roll of a die? Another way to approach the problem of A c B A B S he probability of this event is 5/6 Ac = 1)+2)+3)+4)+6) =5/6 Find the probability of the complement, and subtract from 1 A c = 1 - A c C A c C would mean everything that wasn t in events A or B In this case it is the value of 5 And the probability of rolling a five is 1/6 What is A 1 B for a roll of a die? What is A 1 for a roll of a die? A = [ 2, 4, 6] B = [1, 2, 3] A 1 B = A B 5 S he probability of this event is 1/6 A 1 = 2) 3

4 Additive Rule of Probability (General) A c = A) + A 1 his is called the Additive Rule of Probability (RULE#6 P222) Additive Rule of Probability (in case of mutual exclusive) If events A and B are mutually exclusive, meaning no intersection, then A c =A)+ (RULE#1 P200) wo events are mutual exclusive if when one event occurs in an experiment, the other cannot occur (Def4.4 P198) Example: Events A and A c are mutual exclusive. Roulette is a betting game where a ball spins on a circular wheel that is divided into 38 arcs of equal length Red Numbers Black numbers Green numbers 00 0 You can bet on odd, even, red, black, high, low A: Outcome is an odd number (note 00 and 0 are neither even or odd) A: [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35] A) = 18/38 =.474 B: Outcome is a black number B: [2,4, 6, 8, 10, 11, 13, 15, 17, 20, 22, 24, 26, 28, 29, 31, 33, 35] = 18/38 =.474 C: Outcome is a low number (1-18) C: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18] C) = 18/38 =.474 Find the Intersection of Events A and B A 1 B all numbers that are both odd and black A: [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35] B: [2,4, 6, 8, 10, 11, 13, 15, 17, 20, 22, 24, 26, 28, 29, 31, 33, 35] 4

5 Find the Intersection of Events A and B A 1 B all numbers that are both odd and black A: [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35] B: [2,4, 6, 8, 10, 11, 13, 15, 17, 20, 22, 24, 26, 28, 29, 31, 33, 35] A 1 B = 11, 13, 15, 17 29, 31, 33, 35 Find the Intersection of Events A and C A 1 C all numbers that are both odd and low A: [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35] C: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18] A 1 = 8/38 =.211 Find the Intersection of Events A and C A 1 C all numbers that are both odd and low A: [1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35] C: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18] A 1 C = 1, 3, 5, 7, 9, 11, 13, 15, 17 Find the Intersection of Events B and C B 1 C all numbers that are both black and low B: [2,4, 6, 8, 10, 11, 13, 15, 17, 20, 22, 24, 26, 28, 29, 31, 33, 35] C: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18] B 1 C = 2, 4, 6, 8, 10, 11, 13, 15, 17 A 1 C) = 9/38 =.237 B 1 C) = 9/38 =.237 What about the Intersection between all three Events A 1 B 1 C All odd numbers that are Black and are low 11, 13, 15, 17 A 1 B 1 C) = 4/38 =.105 What about the union between two events? A c B his is all the numbers that are odd or are black Note: there is overlap between them, and they are not mutually exclusive 5

6 All odd numbers and all that are Black 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 2, 4, 6, 8 10, 20, 22, 24, 26, 28 he green numbers (also in bold) represent the overlap A c = 28/38 =.737 We could also use the additive rule A c = A) + - A 1 A c = A c =.737 Note: Because A and B are not mutual exclusive, we must use General Additive Rule. What about A c B c C)? All points that are either odd, Black, or low 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 2, 4, 6, 8 10, 20, 22, 24, 26, 28, 12, 14, 16, 18 32/38 =.842 What about A c B c C)? We can also use the additive rule Only this time it s a bit more complicated A c B c C) = A) + + C) A1 - A 1C) B 1C) + A 1B 1C) = =.842 So what s the point?? Not to make you better gamblers I do want you to understand that probability is based on events within a sample space that defining the probability of an event(s) can be complicated, particularly for compound events here may be more than one way to solve the problem counting, using the formulas and rules, using complements Conditional Probability If we have knowledge that affects the outcome of an experiment, the probabilities will be altered We call this a Conditional Probability (Def4.8 P212) Designated as A he probability of Event A is conditioned on the probability of Event B We often use the term given when talking about conditional probabilities 6

7 Conditional Probability Suppose we have the roll of a die even number) = P[2, 4, 6] = 3/6 = What if we ask the probability of of an even number given the die is less than or equal to 3? even #3) = P[2 #3] = 1/3 Note: it is a 2 out the new or given possible space [1,2,3] Conditional Probability he formula of a conditional probability is: Probability of the intersection of A and B divided by the probability of B A A = It adjusts the probability of the intersection to the reduced sample space of the condition Conditional Probability Back to the die example If A = [even number on a die] B = [less than or equal to 3] A1 = 2) = 1/6 =.1667 = 1) + 2) + 3) = 3/6 = A =.1667/ =.333 or 1/3 Next: Multiplicative Rule he multiplicative rule shows us the probability of an intersection Remember we said a conditional probability is determined by the formula A A = Rearrange terms and we can find the formula for the probability of an intersection between A and B Multiplicative Rule his show the probability of an intersection It suggests that the probability of an intersection between two events depends upon the conditional probability between the two events A = A = A) B A) Probability of an Intersection (Multiplicative Rule) P ( A = A P ( B A) = A) B A) his is Multiplicative Rule (Rule#5 P221). 7

8 Multiplicative Rule and Independence If Events A and B are independent of each other hen, A = A) Likewise, B A) = (Def4.9 P214) Independence means that the probability of A doesn t change given the event B occurs or not Multiplicative Rule in case of Independence In the case of independence between events A and B he formula for probability of an interaction reduces to: A1 = (Rule#4 P214) If we can assume independence between events, figuring the probability of the intersection of events is much easier Look at the Probability of tossing two coins What is the probability of getting two heads? First oss Second oss Earlier we looked at this problem differently Sample space for flipping two coins and noting the face: Observe ¼ =.25 Observe ¼ =.25 Observe ¼ =.25 Observe ¼ =.25 Use Multiplicative Rule In case of Independence We know the first flip is independent to the second flip he probability of observing a head in a single flip of a coin is ½ or he probability of observing a tail in a single flip of a coin is ½ or If I can assume independence two eads) = = ead 1 st ead on the 2 nd flip) = =.25 Multiple through to get the probabilities What is the probability of getting two heads? First oss Second oss x =.25 8

9 Conditional Probability versus Independence A little circular? Conditional probability and independence are very important concepts in research If we hypothesize salary levels differ between men and women, in essence we are saying, given you are a female, I expect your salary is different. If we hypothesize that level of response is different between a drug and the treatment group, we are saying, given you received the drug, your response is higher We often test conditional probability by comparing the data we observe to a hypothetical model of independence Probability of A Union Conditional Probability Probability of an Intersection A = A) + A A A = P ( A = A Example Deck of Cards Four suits, earts, Diamonds, Clubs, Spades 13 in each suit Let Event A = Face Cards (Jack, Queen, King) Let Event B = earts Let Event C = Red suit (earts, Diamonds) Example Define Experiment: selecting from a single Deck of cards List the sample points Event A = [Face Cards] = [ J Q K ; J Q K ; J Q K ; J Q K ] Event B = [earts] = [ A J Q K ] Event C = [Red Suits] = [ A J Q K ; A J Q K ] Assign Probabilities A random draw of each card is 1/52 Event A) = 12* 1/52 = 12/52 =.2308 Event = 13 * 1/52 = 13/52 =.25 Event C) = 26 * 1/52 = 26/52 = What is the Probability of: A c B c C) 9

10 What is the Probability of: A 1 What is the Probability of: A B 1 C) Probability Problem Enterococci are bacteria that cause blood infections in hospitalized patients. One antibiotic used to battle enterococci is vancomycin. A study by the Federal Centers for Disease Control and Prevention revealed that 8% of all enterococci isolated in hospitals nationwide were resistant to vancomycin. Probability Problem Consider a random sample of three patients with blood infections caused by the enterococci bacteria. Assume that all three patients are treated with the antibiotic vancomycin. What is the probability that all three patients are successfully treated? What assumption did you make concerning the patients? What is the probability that the bacteria resist the antibiotic for at least one patient? 10

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