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Section 8.1 - Distributions of Random Variables

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Section 8.1 - Distributions of Random Variables c Kendra Kilmer March 22, 2010 Section 8.1 - Distributions of Random Variables Definition:A random variable is a rule that assigns a number to each outcome of an experiment. Example 1: Suppose we toss a coin three times. Then we could define the random variable X to represent the number of times we get tails. Example 2: Suppose we roll a die until a 5 is facing up. Then we could define the random variable Y to represent the number of times we rolled the die. Example 3: Suppose a flashlight is left on until the battery runs out. Then we could define the random variable Z to represent the amount of time that passed. Types of Random Variables 1. A finite discrete random variable is one which can only take on a limited number of values that can be listed. 2. An infinite discrete random variable is one which can take on an unlimited number of values that can be listed in some sort of sequence. 3. A continuous random variable is one which takes on any of the infinite number of values in some interval of real numbers. (i.e. usually measurements) Example 4: Referring to Example 1, find the probability distribution of the random variable X. 1 c Kendra Kilmer March 22, 2010 Example 5: The number of cars waiting in line at the beginning of 2-minute intervals at a certain bank was observed. The following data was collected: Number of cars 0 1 2 3 4 Frequency of occurence 2 2 16 8 2 Let X represent the number of cars observed waiting in line and find the probability distribution of X We use histograms to represent the probability distributions of random variables. We place the possible values for the random variable X on the horizontal axis. We then center a bar around each x value and let its height be equal to the probability of that x value. Example 6: Referring to Example 5, a) Draw the histogram for the random variable X b) Find P(X ≥ 2) 2 c Kendra Kilmer March 22, 2010 Section 8.2 - Expected Value Example 1: Records kept by the chief dietitian at the university caferteria over a 25-wk period show the following weekly consumption of milk (in gallons): Milk 200 201 202 203 204 Weeks 4 6 8 5 2 Find the average number of gallons of milk consumed per week in the cafeteria. Definition: Let X denote a random variable that assumes the values x1;x2;:::;xn, with associated probabilites p1; p2;:::; pn, respectively. Then the expected value of X, E(X), is given by: E(X) = x1 p1 + x2 p2 + ···xn pn Example 2: Referring to Example 1, let the random variable X denote the number of gallons of milk consumed in a week at the cafeteria. a) Find the probability distribution of X. b) Compute E(X) c) Draw a histogram for the random variable X Note: If we think of placing the histogram on a see-saw, the expected value occurs where we would put the fulcrum to balance it. 3 c Kendra Kilmer March 22, 2010 Example 3: In a lottery, 3000 tickets are sold for $1 each. One first prize of $5000, 1 second prize of $1000, 3 third prizes of $100, and 10 consolation prizes of $10 are to be awarded. What are the expected net earnings of a person who buys one ticket? Example 4: A woman purchased a $15,000 1-year term-life inusrance policy for $200. Assuming that the probability that she will live another year is 0:994, find the company’s expected net gain. 4 c Kendra Kilmer March 22, 2010 Definition:A fair game means that the expected value of the player’s net winnings is zero. (E(X) = 0) Example 5: Mike and Bill play a card game with a standard deck of 52 cards. Mike selects a card from a well- shuffled deck and receives A dollars from Bill if the card selected is a diamond; otherwise, Mike pays Bill one dollar. Determine the value of A if the game is to be fair. Definition: If P(E) is the probability of an event E occuring, then a) The odds in favor of E occuring are P(E) P(E) = [P(E) 6= 1] 1 − P(E) P(Ec) b) The odds against E occuring are 1 − P(E) P(Ec) = [P(E) 6= 0] P(E) P(E) Example 6: The probability of an event E occurring is :8. a) What are the odds in favor of E occurring? b) What are the odds against E occurring? Definition: If the odds in favor of an event E occuring are a to b, then the probability of E occurring is a P(E) = a + b Example 7: If a sports forecaster states that the odds of a certain boxer winning a match are 4 to 3, what is the probability that the boxer will win the match? 5 c Kendra Kilmer March 22, 2010 Definition: The median is the middle value in a set of data arranged in increasing or decreasing order (when there is an odd number of entries). If there is an even number of entries, the median is the average of the two middle numbers. Definition The mode is the value that occurs most frequently in a set of data. Example 8: In each of the following cases, find the mean, median, and mode. 6 c Kendra Kilmer March 22, 2010 Section 8.3 - Variance and Standard Deviation Example 1: Draw the histograms for the random variables X and Y that have the following probability distribu- tions: x P(X = x) y P(Y = y) 1 :1 1 :3 2 :1 2 :1 3 :6 3 :2 4 :1 4 :1 5 :1 5 :3 Definition: Suppose a random variable X has the following probability distribution: x P(X = x) x1 p1 x2 p2 · · · · · · xn pn 2 and expected value E(X) = m. Then the variance of the random variable X is Var(X)=p1(x1 − m) + p2(x2 − 2 2 m) + ··· + pn(xn − m) . Definition: The standard deviation of a random variable X, s, is defined by: p s = Var(X) 7 c Kendra Kilmer March 22, 2010 Finding expected value and standard deviation using the calculator: • Enter the x-values into L1 and the corresponding probabilities into L2 (STAT!1:Edit) • On the homescreen type 1-Var Stats L1 , L2 (STAT!CALC!1:1-Var Stats) • x¯ is the expected value (mean) • sx is the standard deviation • To find variance, you would need to recall that Var(X) = s 2 Example 2: Referring to Example 1, a) Find the variance and standard deviation of X. b) Find the variance and standard deviation of Y. Example 3: The percent of the voting age population who cast ballots in presidential election years from 1980 through 2000 are given below: Election Year 1980 1984 1988 1992 1996 2000 Turnout Percentage 53 53 50 55 49 51 Find the mean and standard deviation of the voter turnout. 8 c Kendra Kilmer March 22, 2010 Chebychev’s Inequality: Let X be a random variable with expected value m and standard deviation s. Then, the 1 probability that a randomly chosen outcome of the experiment lies between m −ks and m +ks is at least 1 − k2 ; that is: 1 P(m − ks ≤ X ≤ m + ks) ≥ 1 − k2 Example 4: A probability distribution has a mean of 10 and a standard deviation of 1:5. Use Chebychev’s inequality to estimate the probability that an outcome of the experiment lies between 7 and 13 Example 5: A probability distribution has a mean of 70 and a standard deviation of 3:2. Use Chebychev’s Inequality to find the value of c that guarantees that the probability is at least 0:984375 that an outcome of the experiment lies between 70 − c and 70 + c. 9 c Kendra Kilmer March 22, 2010 Section 8.4 - The Binomial Distribution Binomial Experiments have the following properties: 1. The number of trials in the experiment is fixed. 2. There are 2 possible outcomes in each trial: “success” and “failure” 3. The probability of success in each trial is the same. 4. Trials are independent of each other. Calculating a Binomial Probability: 1. Determine if the experiment is binomial. 2. Determine the number of trials (n). 3. Define “success” in the experiment and determine the probability of that success occuring (p). 4. Determine the number of “successes” desired (r). 5. Calculate the desired probability: (a) By hand, the probability is found by doing the following calculation: P(X = r) = C(n;r)pr(1 − p)(n−r) (b) We can use the calculator functions binompdf( or binomcdf( to do these calculations for us. Example 1: What is the probability of exactly 3 successes in 6 trials of a binomial experiment in which p = 1=2? Example 2: A fair die is cast 4 times. Compute the probability of obtaining exactly one 6 in the four throws. 10 c Kendra Kilmer March 22, 2010 Example 3: A biology quiz consists of 8 multiple choice questions. Each question has five choices of which only one is correct. If a student randomly guesses on each question, what is the probability of a) getting at most 3 questions right? b) getting at least 2 questions right? c) getting more than 2 but less than 6 questions right? Example 4: Studies have shown that 30% of babies are delivered by C-section.
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