STAT 430/510 Probability Lecture 1: Counting-1

STAT 430/510 Probability Lecture 1: Counting-1 Pengyuan (Penelope) Wang May 22, 2011

Introduction In the early days, probability was associated with games of chance, such as gambling. Probability is describing how likely an event would happen.

Intuitive understanding of probability A fair coin is rolled one. You win $100 if head happens. What is the probability that you win?

Intuitive understanding of probability A fair coin is rolled one. You win $100 if head happens. What is the probability that you win? Solution: 0.5. Why?

Intuitive understanding of probability A fair coin is rolled one. You win $100 if head happens. What is the probability that you win? Solution: 0.5. Why? A fair coin is rolled twice. You win $100 if head happens twice. What is the probability that you win?

Sample Spaces with Equally Likely Outcomes It is useful to have an effective method for counting the number of ways that things can happen. If each outcome is equally likely to happen, then intuitively, the probability is just the number of ways divided by total number of ways. For example: If you roll a die once, intuitively, what is the probability that you get 1?

The Basic Principle of Counting The Basic Principle of Counting Suppose that two experiments are to be performed. Then if experiment 1 can result in anyone of m possible outcomes and if, for each outcome of experiment 1, there are n possible outcomes of experiment 2, then together there are mn possible outcomes of the two experiments.

Example A small community consists of 10 women, each of whom has 3 children. If one woman and one of her children are to be chosen as mother and child of the year, how many different choice are possible? Solution: 10 3 = 30.

The Generalized Basic Principle of Counting The Generalized Basic Principle of Counting If r experiments that are to be performed are such that the first one may result in any of n 1 possible outcomes; and if, for each of these n 1 possible outcomes, there are n 2 possible outcomes of the second experiment; and if, for each of the possible outcomes of the first two experiments, there are n 3 possible outcomes of the third experiment; and if..., then there is a total of n 1 n 2 n r possible outcomes of the r experiments.

Example How many different 7-place license plates are possible if the first 3 places are to be occupied by letters (26 letters) and the final 4 by numbers (10 digits 0,1,...,9)?

Example How many different 7-place license plates are possible if the first 3 places are to be occupied by letters (26 letters) and the final 4 by numbers (10 digits 0,1,...,9)? Solution: 26 26 26 10 10 10 10 = 175760000

Example when repetition among letters (26 letters) or numbers (10 digits 0,1,...,9) were prohibited, how many different 7-place license plates are possible?

Example when repetition among letters (26 letters) or numbers (10 digits 0,1,...,9) were prohibited, how many different 7-place license plates are possible? Solution: 26 25 24 10 9 8 7 = 78624000

Example when repetition among letters (26 letters) or numbers (10 digits 0,1,...,9) were prohibited, how many different 7-place license plates are possible? Solution: 26 25 24 10 9 8 7 = 78624000 If you choose a plate randomly, what is the probability that there is no repetition among letters or numbers??

Permutations: choose k objects from n and order them We want to k distinct objects taken from a set of n objects and ordered them. Such a ordered sequence of k distinct objects taken from a set of n objects is called a permutation of size k. The total number of ways is n(n 1)(n 2) (n k + 1) = n! (n k)!. We denote such number by P k,n = n! (n k)!. Typically, P n,n = n!. It is the number of ways you can order n distinct objects.

Example How many different batting orders are possible for a baseball team consisting of 9 players? Solution: 9! = 362880

Example Ms. Jones has 10 books that she is going to put on her bookshelf. Of these, 4 are mathematical books, 3 are chemistry books, 2 are history books, and 1 is a language book. Ms Jones wants to arrange her books so that all the books dealing with the same subjects are together on the shelf. How many different arrangements are possible?

Example How many different letter arrangements can be formed from the letters EGG? Solution: 3! 2 = 3

Example How many different letter arrangements can be formed from the letters PEPPER?

Example How many different letter arrangements can be formed from the letters PEPPER? Solution: 6! 3!2!1! = 60

Permutations: Continued In general, suppose that we have n objects, of which n 1 are alike, n 2 are alike,..., n k are alike. How many different permutations of the n objects are there? The total number of permutations of the n objects is n! n 1!n 2! n r!

Example How many different signals, each consisting of 9 flags hung in a line, can be made from a set of 4 white flags, 3 red flags, and 2 blue flags if all flags of the same color are identical?

Example How many different signals, each consisting of 9 flags hung in a line, can be made from a set of 4 white flags, 3 red flags, and 2 blue flags if all flags of the same color are identical? Solution: 9! 4!3!2! = 1260

Last Example: Important trick to deal with counting problems: really decompose complex problems into several experiments! (And then use the principle of counting.) How many ways can 8 people be seated in a row if there are no restrictions? A and B must sit next to each other? there are 4 men and 4 women, and no 2 men or 2 women can sit next to each other? there are 4 married couples and they much sit together?