# Applied Statistics I

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1 Applied Statistics I Liang Zhang Department of Mathematics, University of Utah June 12, 2008 Liang Zhang (UofU) Applied Statistics I June 12, / 29

2 In Probability, our main focus is to determine the probabilities for all possible events. However, some prior knowledge about the sample space is available. (While in Statistics, the prior knowledge is unavailable and we want to find it.) Liang Zhang (UofU) Applied Statistics I June 12, / 29

3 In Probability, our main focus is to determine the probabilities for all possible events. However, some prior knowledge about the sample space is available. (While in Statistics, the prior knowledge is unavailable and we want to find it.) For a sample space that is either finite or countably infinite (meaning the outcomes can be listed in an infinite sequence), let E 1, E 2,... denote all the simple events. If we know P(E i ) for each i, then for any event A, P(A) = all E i s that are in A P(E i ) Liang Zhang (UofU) Applied Statistics I June 12, / 29

4 In Probability, our main focus is to determine the probabilities for all possible events. However, some prior knowledge about the sample space is available. (While in Statistics, the prior knowledge is unavailable and we want to find it.) For a sample space that is either finite or countably infinite (meaning the outcomes can be listed in an infinite sequence), let E 1, E 2,... denote all the simple events. If we know P(E i ) for each i, then for any event A, P(A) = all E i s that are in A P(E i ) Here the knowledge for P(E i ) is given and we want to find P(A). (In statistics, we want to find the knowledge about P(E i ).) Liang Zhang (UofU) Applied Statistics I June 12, / 29

5 Example 2.15: Liang Zhang (UofU) Applied Statistics I June 12, / 29

6 Example 2.15: During off-peak hours a commuter train has five cars. Suppose a commuter is twice as likely to select the middle car(#3) as to select either adjacent car(#2 or #4) (1), and is twice as likely to select either adjacent car as to select either end car(#1 or #5) (2). What is the probability for the commuter to select one of the three middle cars? Liang Zhang (UofU) Applied Statistics I June 12, / 29

7 Example 2.15: During off-peak hours a commuter train has five cars. Suppose a commuter is twice as likely to select the middle car(#3) as to select either adjacent car(#2 or #4) (1), and is twice as likely to select either adjacent car as to select either end car(#1 or #5) (2). What is the probability for the commuter to select one of the three middle cars? First we need to determine the probabilities for all the simple events: Liang Zhang (UofU) Applied Statistics I June 12, / 29

8 Example 2.15: During off-peak hours a commuter train has five cars. Suppose a commuter is twice as likely to select the middle car(#3) as to select either adjacent car(#2 or #4) (1), and is twice as likely to select either adjacent car as to select either end car(#1 or #5) (2). What is the probability for the commuter to select one of the three middle cars? First we need to determine the probabilities for all the simple events: Let p i = P({car i is selected}) = P(E i ). Liang Zhang (UofU) Applied Statistics I June 12, / 29

9 Example 2.15: During off-peak hours a commuter train has five cars. Suppose a commuter is twice as likely to select the middle car(#3) as to select either adjacent car(#2 or #4) (1), and is twice as likely to select either adjacent car as to select either end car(#1 or #5) (2). What is the probability for the commuter to select one of the three middle cars? First we need to determine the probabilities for all the simple events: Let p i = P({car i is selected}) = P(E i ). Then condition (1) tells us p 3 = 2p 2 = 2p 4, Liang Zhang (UofU) Applied Statistics I June 12, / 29

10 Example 2.15: During off-peak hours a commuter train has five cars. Suppose a commuter is twice as likely to select the middle car(#3) as to select either adjacent car(#2 or #4) (1), and is twice as likely to select either adjacent car as to select either end car(#1 or #5) (2). What is the probability for the commuter to select one of the three middle cars? First we need to determine the probabilities for all the simple events: Let p i = P({car i is selected}) = P(E i ). Then condition (1) tells us p 3 = 2p 2 = 2p 4, and condition (2) tells us p 2 = 2p 1 = 2p 5 = p 4. Liang Zhang (UofU) Applied Statistics I June 12, / 29

11 Example 2.15: During off-peak hours a commuter train has five cars. Suppose a commuter is twice as likely to select the middle car(#3) as to select either adjacent car(#2 or #4) (1), and is twice as likely to select either adjacent car as to select either end car(#1 or #5) (2). What is the probability for the commuter to select one of the three middle cars? First we need to determine the probabilities for all the simple events: Let p i = P({car i is selected}) = P(E i ). Then condition (1) tells us p 3 = 2p 2 = 2p 4, and condition (2) tells us p 2 = 2p 1 = 2p 5 = p 4. By Axiom 2 and 3, 1 = 5 P(E i ) = p 1 + 2p 1 + 4p 1 + 2p 1 + p 1 = 10p 1 i=1 Liang Zhang (UofU) Applied Statistics I June 12, / 29

12 Example 2.15 (continued): Since p 3 = 2p 2 = 2p 4, p 2 = 2p 1 = 2p 5 = p 4 and 1 = 5 i=1 P(E i) = p 1 + 2p 1 + 4p 1 + 2p 1 + p 1 = 10p 1, Liang Zhang (UofU) Applied Statistics I June 12, / 29

13 Example 2.15 (continued): Since p 3 = 2p 2 = 2p 4, p 2 = 2p 1 = 2p 5 = p 4 and 1 = 5 i=1 P(E i) = p 1 + 2p 1 + 4p 1 + 2p 1 + p 1 = 10p 1, we have p 1 = 0.1, p 2 = 0.2, p 3 = 0.4, p = 0.2 and p 5 = 0.1. Liang Zhang (UofU) Applied Statistics I June 12, / 29

14 Example 2.15 (continued): Since p 3 = 2p 2 = 2p 4, p 2 = 2p 1 = 2p 5 = p 4 and 1 = 5 i=1 P(E i) = p 1 + 2p 1 + 4p 1 + 2p 1 + p 1 = 10p 1, we have p 1 = 0.1, p 2 = 0.2, p 3 = 0.4, p = 0.2 and p 5 = 0.1. Furthermore if A = {one of the three middle cars is selected}, then P(A) = P(E 2 ) + P(E 3 ) + P(E 4 ) = p 2 + p 3 + p 4 = 0.8. Liang Zhang (UofU) Applied Statistics I June 12, / 29

15 Equally Likely Outcomes: experiments whose outcomes have exactly the same probabilitiy. Liang Zhang (UofU) Applied Statistics I June 12, / 29

16 Equally Likely Outcomes: experiments whose outcomes have exactly the same probabilitiy. In that case, the probability p for each simple event E i is determined by the size of the sample space N, i.e. p = P(E i ) = 1 N Liang Zhang (UofU) Applied Statistics I June 12, / 29

17 Equally Likely Outcomes: experiments whose outcomes have exactly the same probabilitiy. In that case, the probability p for each simple event E i is determined by the size of the sample space N, i.e. This is simply due to the fact p = P(E i ) = 1 N N 1 = P(S) = P( E i ) = i=1 N P(E i ) = i=1 N p = N p i=1 Liang Zhang (UofU) Applied Statistics I June 12, / 29

18 Examples: Liang Zhang (UofU) Applied Statistics I June 12, / 29

19 Examples: tossing a fair coin: N = 2 and P({H}) = P({T }) = 1/2; Liang Zhang (UofU) Applied Statistics I June 12, / 29

20 Examples: tossing a fair coin: N = 2 and P({H}) = P({T }) = 1/2; tossing a fair die: N = 6 and P({1}) = P({2}) = P({3}) = P({4}) = P({5}) = P({6}) = 1/6; Liang Zhang (UofU) Applied Statistics I June 12, / 29

21 Examples: tossing a fair coin: N = 2 and P({H}) = P({T }) = 1/2; tossing a fair die: N = 6 and P({1}) = P({2}) = P({3}) = P({4}) = P({5}) = P({6}) = 1/6; randomly selecting a student from 25 students: N = 25 and p = 1/25. Liang Zhang (UofU) Applied Statistics I June 12, / 29

22 Counting Techniques: Liang Zhang (UofU) Applied Statistics I June 12, / 29

23 Counting Techniques: If the sample space is finite and all the outcomes are equally likely to happen, then the formula P(A) = P(E i ) simplifies to all E i s that are in A P(A) = N(A) N where E i is any simple event, N is the number of outcomes of the sample space and N(A) is the number of outcomes contained in event A. Liang Zhang (UofU) Applied Statistics I June 12, / 29

24 Counting Techniques: If the sample space is finite and all the outcomes are equally likely to happen, then the formula P(A) = P(E i ) simplifies to all E i s that are in A P(A) = N(A) N where E i is any simple event, N is the number of outcomes of the sample space and N(A) is the number of outcomes contained in event A. Determining the probability of A counting N(A). Liang Zhang (UofU) Applied Statistics I June 12, / 29

25 Product Rule for Ordered Pairs: Liang Zhang (UofU) Applied Statistics I June 12, / 29

26 Product Rule for Ordered Pairs: Proposition If the first element of object of an ordered pair can be selected in n 1 ways, and for each of these n 1 ways the second element of the pair can be selected in n 2 ways, then the number of pairs is n 1 n 2. Liang Zhang (UofU) Applied Statistics I June 12, / 29

27 Examples: Liang Zhang (UofU) Applied Statistics I June 12, / 29

28 Examples: Suppose a coin is tossed and then a marble is selected at random from a box containing one black (B), one red (R), and one green (G) marble. The possible outcomes are HB, HR, HG, TB, TR and TG. For each of the two possible outcomes of the coin there are three marbles that may be selected for a total of 2 3 = 6 possible outcomes. Liang Zhang (UofU) Applied Statistics I June 12, / 29

29 Examples: Suppose a coin is tossed and then a marble is selected at random from a box containing one black (B), one red (R), and one green (G) marble. The possible outcomes are HB, HR, HG, TB, TR and TG. For each of the two possible outcomes of the coin there are three marbles that may be selected for a total of 2 3 = 6 possible outcomes. We can also use the tree diagram to illustrate: Liang Zhang (UofU) Applied Statistics I June 12, / 29

30 Examples: Liang Zhang (UofU) Applied Statistics I June 12, / 29

31 Examples: Suppose we draw two cards from a deck of 52 cards. Each time, we record the suit of that card and then replace it. The outcome for each drawing is hearts ( ), diamonds ( ), clubs ( ) and spades ( ). Liang Zhang (UofU) Applied Statistics I June 12, / 29

32 Examples: Suppose we draw two cards from a deck of 52 cards. Each time, we record the suit of that card and then replace it. The outcome for each drawing is hearts ( ), diamonds ( ), clubs ( ) and spades ( ). If we are only interested in getting spades ( ), then there are 13 different outcomes for each drawing and the total number of outcomes with is = 169. Liang Zhang (UofU) Applied Statistics I June 12, / 29

33 Examples: Suppose we draw two cards from a deck of 52 cards. Each time, we record the suit of that card and then replace it. The outcome for each drawing is hearts ( ), diamonds ( ), clubs ( ) and spades ( ). If we are only interested in getting spades ( ), then there are 13 different outcomes for each drawing and the total number of outcomes with is = 169. However, if we do not replace the card after our first drawing, then there are still 13 outcomes with a for the first drawing but 12 outcomes with a for the second drawing. This time the total number of outcomes with is = 156. Liang Zhang (UofU) Applied Statistics I June 12, / 29

34 General Product Rule: Liang Zhang (UofU) Applied Statistics I June 12, / 29

35 General Product Rule: A k-tuple is an ordered collection of k objects. Liang Zhang (UofU) Applied Statistics I June 12, / 29

36 General Product Rule: A k-tuple is an ordered collection of k objects. e.g. (2,3,1,6) from tossing a die 4 times; (,, ) from drawing cards; etc. Liang Zhang (UofU) Applied Statistics I June 12, / 29

37 General Product Rule: A k-tuple is an ordered collection of k objects. e.g. (2,3,1,6) from tossing a die 4 times; (,, ) from drawing cards; etc. Proposition Suppose a set consists of ordered collections of k elements (k-tuples) and that there are n 1 possible choices for the first element; for each choice of the first element, there n 2 possible choices of the second element;... ; for each possible choice of the first k 1 elements, there are n k choices of the k th element. Then there are n 1 n 2 n k possible k-tuples. Liang Zhang (UofU) Applied Statistics I June 12, / 29

38 Examples: We toss a coin 6 times. Then any outcome will be a 6-tuple. For example, (H,H,T,H,T,T). For each toss, we have two possibilities. Therefore the total number of outcomes would be = 2 6 = 64 Liang Zhang (UofU) Applied Statistics I June 12, / 29

39 Examples: We toss a coin 6 times. Then any outcome will be a 6-tuple. For example, (H,H,T,H,T,T). For each toss, we have two possibilities. Therefore the total number of outcomes would be = 2 6 = 64 If we restrict our attention to those outcomes starting with H, then the total number of outcomes would be = 2 5 = 32 Liang Zhang (UofU) Applied Statistics I June 12, / 29

40 Permutations and Combinations Liang Zhang (UofU) Applied Statistics I June 12, / 29

41 Permutations and Combinations Example: Assume we are going to organize a club for learning SAS and all 25 students here will be the members. We need to elect someone to be the president, the vice-president and the treasurer. How many choices do we have? Liang Zhang (UofU) Applied Statistics I June 12, / 29

42 Permutations and Combinations Example: Assume we are going to organize a club for learning SAS and all 25 students here will be the members. We need to elect someone to be the president, the vice-president and the treasurer. How many choices do we have? If there is a university meeting and we need to find 3 members to present our activities, how many choices do we have to find the 3 representives? Liang Zhang (UofU) Applied Statistics I June 12, / 29

43 Permutations and Combinations Example: Assume we are going to organize a club for learning SAS and all 25 students here will be the members. We need to elect someone to be the president, the vice-president and the treasurer. How many choices do we have? If there is a university meeting and we need to find 3 members to present our activities, how many choices do we have to find the 3 representives? For the first question, the order is important while in the second question, the order doesn t make any difference. Liang Zhang (UofU) Applied Statistics I June 12, / 29

44 Permutation: an ordered subset is called a permutation. Liang Zhang (UofU) Applied Statistics I June 12, / 29

45 Permutation: an ordered subset is called a permutation. We use P k:n to denote the number of permutations of size k that can be formed from n individuals. Liang Zhang (UofU) Applied Statistics I June 12, / 29

46 Permutation: an ordered subset is called a permutation. We use P k:n to denote the number of permutations of size k that can be formed from n individuals. e.g. selecting 3 digits from {1, 2, 3, 4, 5, 6} to form a 3-digit number; etc. Liang Zhang (UofU) Applied Statistics I June 12, / 29

47 Permutation: an ordered subset is called a permutation. We use P k:n to denote the number of permutations of size k that can be formed from n individuals. e.g. selecting 3 digits from {1, 2, 3, 4, 5, 6} to form a 3-digit number; etc. Combination: an unordered subset is called a combination. Liang Zhang (UofU) Applied Statistics I June 12, / 29

48 Permutation: an ordered subset is called a permutation. We use P k:n to denote the number of permutations of size k that can be formed from n individuals. e.g. selecting 3 digits from {1, 2, 3, 4, 5, 6} to form a 3-digit number; etc. Combination: an unordered subset is called a combination. We use ( n k) to denote the number of combinations for choosing k individuals from n individuals. Liang Zhang (UofU) Applied Statistics I June 12, / 29

49 Permutation: an ordered subset is called a permutation. We use P k:n to denote the number of permutations of size k that can be formed from n individuals. e.g. selecting 3 digits from {1, 2, 3, 4, 5, 6} to form a 3-digit number; etc. Combination: an unordered subset is called a combination. We use ( n k) to denote the number of combinations for choosing k individuals from n individuals. e.g. selecting 3 digits from {1, 2, 3, 4, 5, 6} to form a size 3 subset; etc. Liang Zhang (UofU) Applied Statistics I June 12, / 29

50 Proposition P k:n = n (n 1) (n (k 1)) = where k! = k (k 1) 2 1 is the k factorial. n! (n k)! Liang Zhang (UofU) Applied Statistics I June 12, / 29

51 Proposition P k:n = n (n 1) (n (k 1)) = where k! = k (k 1) 2 1 is the k factorial. n! (n k)! Examples: Assume we are going to organize a club for learning SAS and all 25 students here will be the members. We need to elect someone to be the president, the vice-president and the treasurer. How many choices do we have? Liang Zhang (UofU) Applied Statistics I June 12, / 29

52 Proposition ( ) n = P k:n k k! = n! k!(n k)! where k! = k (k 1) 2 1 is the k factorial. Liang Zhang (UofU) Applied Statistics I June 12, / 29

53 Proposition ( ) n = P k:n k k! = n! k!(n k)! where k! = k (k 1) 2 1 is the k factorial. Examples: Assume we are going to organize a club for learning SAS and all 25 students here will be the members. If there is a university meeting and we need to find 3 members to present our activities, how many choices do we have to find the 3 representives? Liang Zhang (UofU) Applied Statistics I June 12, / 29

54 Permutations and Combinations Examples: Liang Zhang (UofU) Applied Statistics I June 12, / 29

55 Permutations and Combinations Examples: Example 2.23 A university warehouse has received a shipment of 25 printers, of which 10 are laser printers and 15 are inkjet models. If 6 of these 25 are selected at random to be checked by a particular technician, what is the probability that exactly 3 of those selected are laser printers? Liang Zhang (UofU) Applied Statistics I June 12, / 29

56 Permutations and Combinations Examples: Example 2.23 A university warehouse has received a shipment of 25 printers, of which 10 are laser printers and 15 are inkjet models. If 6 of these 25 are selected at random to be checked by a particular technician, what is the probability that exactly 3 of those selected are laser printers? What is the probability that at least 3 of those selected are laser printers? Liang Zhang (UofU) Applied Statistics I June 12, / 29

57 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

58 Conditional Probability Example 2.24 Complex components are assembled in a plant that uses two different assembly lines, A and B. Line A uses older equipment than B, so it is somewhat slower and less reliable. Suppose on a given day line A has assembled 8 components, of which 2 have been identified as defective (D) and 6 as nondefective (ND), whereras B has produced 1 defective and 9 nondefective components. This information is summarized in the following table. Line Condition D ND A 2 6 B 1 9 Unaware of this information, the sales manager randomly selects 1 of these 18 components for a demonstration. Liang Zhang (UofU) Applied Statistics I June 12, / 29

59 Conditional Probability Line Condition D ND A 2 6 B 1 9 P({line A component selected}) = N(A) N = 8 18 = 4 9 Liang Zhang (UofU) Applied Statistics I June 12, / 29

60 Conditional Probability Line Condition D ND A 2 6 B 1 9 P({line A component selected}) = N(A) N = 8 18 = 4 9 However, if the chosen component turns out to be defective, then it s more likely for the component to be produced by line A. That s because we are now focusing on column D and the component must have been 1 of 3 in the D column. Mathematically speaking, P(A D) = 2 3 = = P(A D) PD Liang Zhang (UofU) Applied Statistics I June 12, / 29

61 Conditional Probability Line Condition D ND A 2 6 B 1 9 P(A) = 4 9 = P(A D) = 2 3 Because we have some prior knowledge that the selected component is defective. Liang Zhang (UofU) Applied Statistics I June 12, / 29

62 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

63 Conditional Probability Definition For any two events A and B with P(B) > 0, the conditional probability of A given that B has occurred is defined by P(A B) = P(A B) P(B) Liang Zhang (UofU) Applied Statistics I June 12, / 29

64 Conditional Probability Definition For any two events A and B with P(B) > 0, the conditional probability of A given that B has occurred is defined by P(A B) = P(A B) P(B) Event B is the prior knowledge. Due to the presence of event B, the probability for event A to happen changed. Liang Zhang (UofU) Applied Statistics I June 12, / 29

65 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

66 Conditional Probability Example: A maintenance firm has gathered the following information regarding the failure mechanisms for air conditioning systems: evidence of gas leaks yes no evidence of yes electrical failure no 32 3 The units without evidence of gas leaks or electrical failure showed other types of failure. If this is a representative sample of AC failure, find the probability for (a)there is evidence of electrical failure given that there was a gas leak; (b)there is evidence of a gas leak given that there is evidence of electrical failure. Liang Zhang (UofU) Applied Statistics I June 12, / 29

67 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

68 Conditional Probability The Multiplication Rule P(A B) = P(A B) P(B) Liang Zhang (UofU) Applied Statistics I June 12, / 29

69 Conditional Probability The Multiplication Rule P(A B) = P(A B) P(B) This is obtained directly from the definition of conditional probability: P(A B) = P(A B) P(B) Liang Zhang (UofU) Applied Statistics I June 12, / 29

70 Conditional Probability The Multiplication Rule P(A B) = P(A B) P(B) This is obtained directly from the definition of conditional probability: P(A B) = P(A B) P(B) Sometimes, we know the probability for event A to happen conditioned on the presence of event B, then we can use the multiplication rule to calculate the probability that event A and B happening simultaneously. Liang Zhang (UofU) Applied Statistics I June 12, / 29

71 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

72 Conditional Probability Example 2.27 Four individuals have responded to a request by a blood bank for blood donations. None of them has donated before, so their blood types are unknown. Suppose only type O+ is desired and only one of the four actually has this type. If the potential donors are selected in random order for typing, what is the probability that at least three individuals must be typed to obtain the desired type? Liang Zhang (UofU) Applied Statistics I June 12, / 29

73 Conditional Probability Example 2.27 Four individuals have responded to a request by a blood bank for blood donations. None of them has donated before, so their blood types are unknown. Suppose only type O+ is desired and only one of the four actually has this type. If the potential donors are selected in random order for typing, what is the probability that at least three individuals must be typed to obtain the desired type? What is the probability that exactly three individuals are typed to obtain the desired type? Liang Zhang (UofU) Applied Statistics I June 12, / 29

74 Conditional Probability Example 2.27 Four individuals have responded to a request by a blood bank for blood donations. None of them has donated before, so their blood types are unknown. Suppose only type O+ is desired and only one of the four actually has this type. If the potential donors are selected in random order for typing, what is the probability that at least three individuals must be typed to obtain the desired type? What is the probability that exactly three individuals are typed to obtain the desired type? Remark: the multiplication rule can be extended to multiple events: P(A 1 A 2 A 3 ) = P(A 3 A 1 A 2 ) P(A 1 A 2 ) = P(A 3 A 1 A 2 ) P(A 2 A 1 ) P(A 1 ) Liang Zhang (UofU) Applied Statistics I June 12, / 29

75 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

76 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

77 Conditional Probability The Law of Total Probability Let A 1, A 2,..., A k be mutually exclusive and exhaustive events. Then for any other event B, P(B) = P(B A 1 ) P(A 1 ) + P(B A 2 ) P(A 2 ) + + P(B A k ) P(A k ) k = P(B A i ) P(A i ) i=1 where exhaustive means A 1 A 2 A k = S. Liang Zhang (UofU) Applied Statistics I June 12, / 29

78 Conditional Probability The Law of Total Probability Let A 1, A 2,..., A k be mutually exclusive and exhaustive events. Then for any other event B, P(B) = P(B A 1 ) P(A 1 ) + P(B A 2 ) P(A 2 ) + + P(B A k ) P(A k ) k = P(B A i ) P(A i ) i=1 where exhaustive means A 1 A 2 A k = S. Liang Zhang (UofU) Applied Statistics I June 12, / 29

79 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

80 Conditional Probability Bayes Theorem Let A 1, A 2,..., A k be a collection of k mutually exclusive and exhaustive events with prior probabilities P(A i )(i = 1, 2,..., k). Then for any other event B with P(B) > 0, the posterior probability of A j given that B has occurred is P(A j B) = P(A j B) P(B) = P(B A j ) P(A j ) k i=1 P(B A i) P(A i ) j = 1, 2,... k Liang Zhang (UofU) Applied Statistics I June 12, / 29

81 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

82 Conditional Probability Application of Bayes Theorem Example 2.30 Incidence of a rare disease Only 1 in 1000 adults is afflicted with a rare disease for which a diagnostic test has been developed. The test is such that when an individual actually has the disease, a positive result will occur 99% of the time, whereas an individual without the disease will show a positive test result only 2% of the time. If a randomly selected individual is tested and the result is positive, what is the probability that the individual has the disease? Liang Zhang (UofU) Applied Statistics I June 12, / 29

83 Conditional Probability Liang Zhang (UofU) Applied Statistics I June 12, / 29

84 Conditional Probability Example 2.29 A chain of video stores sells three different brands of DVD players. Of its DVD player sales, 50% are brand 1, 30% are brand 2, and 20% are brand 3. Each manufacturer offers a 1-year warranty on parts and labor. It is known that 25% of brand 1 s DVD players require warranty on parts and labor, whereas the corresponding percentages for brands 2 and 3 are 20% and 10%, respectively. 1. What is the probability that a randomly selected purchaser has bought a brand 1 DVD player that will need repair while under warranty? Liang Zhang (UofU) Applied Statistics I June 12, / 29

85 Conditional Probability Example 2.29 A chain of video stores sells three different brands of DVD players. Of its DVD player sales, 50% are brand 1, 30% are brand 2, and 20% are brand 3. Each manufacturer offers a 1-year warranty on parts and labor. It is known that 25% of brand 1 s DVD players require warranty on parts and labor, whereas the corresponding percentages for brands 2 and 3 are 20% and 10%, respectively. 1. What is the probability that a randomly selected purchaser has bought a brand 1 DVD player that will need repair while under warranty? 2. What is the probability that a randomly selected purchaser has a DVD player that will need repair while under warranty? Liang Zhang (UofU) Applied Statistics I June 12, / 29

86 Conditional Probability Example 2.29 A chain of video stores sells three different brands of DVD players. Of its DVD player sales, 50% are brand 1, 30% are brand 2, and 20% are brand 3. Each manufacturer offers a 1-year warranty on parts and labor. It is known that 25% of brand 1 s DVD players require warranty on parts and labor, whereas the corresponding percentages for brands 2 and 3 are 20% and 10%, respectively. 1. What is the probability that a randomly selected purchaser has bought a brand 1 DVD player that will need repair while under warranty? 2. What is the probability that a randomly selected purchaser has a DVD player that will need repair while under warranty? 3. If a customer returns to the store with a DVD player that needs warranty work, what is the probability that it is a brand 1 DVD player? A brand 2 DVD player? A brand 3 DVD player? Liang Zhang (UofU) Applied Statistics I June 12, / 29

87 Permutations and Combinations Examples: Liang Zhang (UofU) Applied Statistics I June 12, / 29

88 Permutations and Combinations Examples: A photographer is going to take a photo for 9 students. r of the students are girls and the other 5 are boys. The photographer requires the students to stand in one row and girls should not be adjacent. How many choices could the photographer make? Liang Zhang (UofU) Applied Statistics I June 12, / 29

89 Permutations and Combinations Examples: A photographer is going to take a photo for 9 students. r of the students are girls and the other 5 are boys. The photographer requires the students to stand in one row and girls should not be adjacent. How many choices could the photographer make? boy boy boy boy boy Liang Zhang (UofU) Applied Statistics I June 12, / 29

90 Permutations and Combinations Examples: A photographer is going to take a photo for 9 students. r of the students are girls and the other 5 are boys. The photographer requires the students to stand in one row and girls should not be adjacent. How many choices could the photographer make? boy boy boy boy boy If girls are allowed to be adjacent, but cannot form concecutive 3, how many choices could the photographer make? Liang Zhang (UofU) Applied Statistics I June 12, / 29

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