# Statistics 100A Homework 4 Solutions

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1 Chapter 4 Statistics 00A Homework 4 Solutions Ryan Rosario 39. A ball is drawn from an urn containing 3 white and 3 black balls. After the ball is drawn, it is then replaced and another ball is drawn. This goes on indefinitely. What is the probability that of the first 4 balls drawn, exactly are white? First, note that we have a finite number of trials, n 4 although the game goes on forever, we are only concerned with the first 4 balls. Each trial is a Bernoulli trial - that is, each trial has only one of two outcomes: white and not white. Define a success as the event that a white ball is drawn. Then the probability of success p is p. Since each ball is replaced after it is drawn, we have sampling with replacement, and thus independence. Since we are dealing with a finite number of independent Bernoulli trials with a constant probability of success p, we use the binomial distribution. Let X be the number of white balls successes that appear in n 4 trials. Then we want to find P X. Recall that Then, P X k n k p k p n k P X A man claims to have extrasensory perception. As a test, a fair coin is flipped 0 times and the man is asked to predict the outcome in advance. He gets 7 out of 0 correct. What is the probability that he would have done at least this well if he had no ESP? At first this problem may seem confusing. We have n 0 trials and each flip of a coin is independent. But what is the probability of success? Take a closer look at the problem. What is the probability that he would have done at least this well if he had no ESP? We solve this problem on the premise that he does not have ESP. Therefore, p, the probability of randomly guessing the outcome. We define a success as the event that the man guesses correctly.

2 Since we have a finite number of independent Bernoulli trials with constant probability of success, we use the binomial. Let X be the number of times the man guesses the outcome correctly the number of successes. We want to know the probability that he does at least as well as he did in the observed study where he guessed correctly 7 times out of 0. Then, P X 7 P X 7 + P X 8 + P X 9 + P X [ ] If the man did not have ESP, then we would expect him to guess at least 7 out of the 0 outcomes correctly approximately 7.% of the time. For those that have taken introductory statistics, this problem can be formulated as a hypothesis test. The null hypothesis is that the man does not have ESP and the probabiliity of guessing an outcome correctly is 0.5. The alternative hypothesis is that the man has ESP and can guess correctly more than 50% of the time. That is, { H0 : p 0.5 H a : p > 0.5 The probability we computed in this problem is called a p-value. The p-value in this context is the probability that we observe a sample proportion at least as extreme as the value we observed ˆp 0.7 assuming that the null hypothesis H 0 is true. The hypothesis test that we could have performed would have been the binomial test, not a proportions test, because the success/failure condition fails.

3 48. It is known that diskettes produced by a certain company will be defective with probability.0, independent of each other. The company sells the diskettes in packages of size 0 and offers a money-back guarantee that at most 0f the 0 diskettes in the package will be defective. If someone buys 3 packages, what is the probability that he or she will return exactly of them? Consider this problem hierarchically. There are two levels to this problem that must be solved: within package, and between packages. Level : Within Package First, let s find the probability that we return an arbitrary package. We return the package if more than diskette is defective. Each package contains 0 independent diskettes, that is, n 0. The probability that a diskette is defective is constant, p 0.0. Since p is small, we could use the Poisson, but we use the Binomial since the problem does not ask for an approximate probability. If we were to use the Poisson, we would fix the mean, λ np P X > P X P X P X 0 e e ! 0!.e Using the binomial is the preferred method. P X > P X P X P X The probability of returning an arbitrary package of 0 diskettes is Level : Between Packages We have n 3 packages, which should be independent. We know that the probability of returning an arbitary 0-pack of diskettes is and this is the probability of success, p and it is constant. Let X be the number of packages out of 3 that we return. We want to find P X. By the binomial model, P X

4 5. The monthly worldwide average number of airplane crashes of commercial airlines is 3.5. What is the probability that there will be First we need to decide which probability model to use. All we know is the mean, which does not tell us much. However, the context of the problem implies which model to use. An airline crash is very rare, thus it has a small probability of occurring. We use the Poisson distribution for this situation. For the Poisson distribution, recall that P X i e λ λ i i! Since the mean is 3.5, and the mean is λ, we know that λ 3.5. a at least such accidents in the next month? P X P X P X 0 e e ! 0! 3.5e 3.5 e e b at most accident in the next month? P X P X 0 + P X e e !! 4.5e

5 53. Approximately 80,000 marriages took place in the state of New York last year. Estimate the probability that for at least one of these couples a both partners were born on April 30. The probability that both partners share the same birthday can be found by independence: P Same Birthday, April which is a tiny probability, so we use the Poisson approximation to the binomial. The number of couples that share the same birthday, April 30, is Poisson with mean λ np 80, Then, let X be the number of couples that share April 30 as their birthday. P X P X 0 e ! e b both partners celebrated their birthday on the same day of the year. This problem is different! It is a generalization of part a. Now want to find the probability that both partners celebrated their birthday on the same day of the year, but not just April 30. The computation is similar, however, we note that we can choose from one of 365 days of the year for both partners, P Same Birthday Again, this probability is tiny so we use the Poisson with mean λ np 80, Then, let X be the number of couples that share the same birthday. P X P X 0 e ! e 9. 5

6 59. If you buy a lottery ticket in 50 lotteries, in each of which your chance of winning a prize is, what is the approximate probability that you will win a prize 00 We have a finite number of independent trials n 50 with a given constant probability of success winning a prize p 00 which is small. We could use the binomial model for an exact answer, or the Poisson model for an approximate answer. Since the problem asks for an approximation, we use the Poisson. a at least once? Using the Poisson approximation, P X P X 0 e 0 0! e b exactly once? P X e! e c at least twice? P X P X 0 P X } {{ } } {{ } Part a Part b Consider a roulette wheel consisting of numbers - through 36, 0 and double 0. If Smith always bets that the outcome will be one of the numbers through, what is the probability that Since Smith always bets on the numbers through, which occupy spaces on the wheel, the probability of success winning is p. a Smith will lose his first 5 bets? The first five bets form a finite set of n 5 trials. Each spin of the roulette wheel is independent, and the probability of success p is constant. We use the binomial model. We could also solve this problem intuitively by using independence. 6

7 Let W denote the event that Smith wins his bet. Using independence, the probability that Smith loses his first 5 bets is, P W W W W W Using the binomial, let X be the number of bets that Smith wins. Then, P X b his first win will occur on his fourth bet? There are three equivalent ways to formulate this problem. First, let s tackle this problem intuitively. Smith s first win occurring on the fourth bet means that he loses the first 3 bets and wins on the fourth bet. That is, P W W W W We could also solve this problem using the Geometric distribution. number of the trial of which the first success win occurs. Then, Let X be the P X i p i p Thus, P X We could also interpret the problem as follows the nth trial is the rth win. We can use the negative binomial distribution. But since r, it reduces to the geometric. Let X be the number of trials required to obtain the rth success, then P X n n r p r p n r, n r, r +,... 7

8 Let n 4, r. P X Solve the Banach match problem when the left-hand matchbox originally contained N matches and the right-hand box contained N matches. First, consider the case where the right matchbox is the first to run out of matches. We started with N matches and ended up with 0. The left matchbox started with N matches and ended up with k. We note that the N + st match is drawn from the right matchbox on the N + + N kth trial. Thus, n N + N + k, r N + and p. By using the negative binomial model, we get the following if we assume that E is the event that the right matchbox is the first one discovered empty, P E N + N k N N + N k N N + N +N k+ N +N + k N + But that is only one case. Now consider the case that the left matchbook is the first one discovered empty. Denote this event E. We started with N matches and ended up with 0. The right matchbox started with N matches and ended up with k. We note that the N + st match is drawn from the left matchbox on the N + + N kth trial. Thus, n N + N + k, r N + and p. Then, P E N + N k N N + N k N N + N +N k+ N +N + k N + The events E and E are disjoint so we can add the probabilities together to yield the total probability for this variation of the Banach match problem. P E + P E N + N k N N +N k+ N + N + k N N +N k+ 8

9 79. Suppose that a batch of 00 items contains 6 that are defective and 94 that are non-defective. If X is the number of defective items in a randomly drawn sample of 0 items from the batch, find This is a classic example of the hypergeometric model. We have some population of N items, m of which are hot and we draw a sample of size n from this population. We want to find the probability that the drawn sample of n contains X hot items. Let X be the number of hot items drawn. In this context, N 00, n 0, m 6. P X i m i N m n i N n a P X P X b P X > P X > P X 0 P X P X

10 Chapter 4 Theoretical Exercises 35. An urn initially contains one red and one blue ball. At each stage a ball is randomly chosen and then replaced along with another of the same color. Let X denote the selection number of the first chosen ball that is blue. For instance, if the first selection is red and the second blue, then X is equal to. First, we must understand what is going on here. The urn contains a red ball and a blue ball. We draw a ball and note its color. Then we replace the ball and add a new ball of the same color. a Find P X > i, i. This problem is difficult. We want to find P X > i, that is, the probability that the first draw of a blue ball is after the ith trial. All we know from this is that on the first i draws we drew a red ball. On trial i, there is a red ball and a blue ball so the probability of drawing a red ball is. On trial i, there is an additional red ball added into the urn from draw, so the probability of drawing a red ball on the second draw after drawing a red ball on the first draw is 3. Then we replace this red ball and add in another red ball; so for draw i 3 there are 4 balls, 3 which are red, so the probability of drawing a red ball after already drawing red balls is 3 i 4. Therefore, on the ith trial, there is a probability of i+ of drawing a red ball after having drawn red balls on each of the i prior trials. Thus, P X > i i i i i + Note that most stuff cancels here, leaving us with the answer! P X > i i i i i + i+ There is a more intuitive way to solve this problem. Recall that P X > i P X P X P X < i We can solve this problem for several values of i and notice a pattern. X is the case that we draw a blue ball on the first draw. Since there are balls, red and blue, the probability of drawing a blue ball on the first draw is simply. P X > i P X P X < i } {{ } 0 0

11 X is the case that the first blue ball drawn is chosen on the second round. This means the first draw was red which occurs with probability and the second is blue. However, on the first draw, after we put the red ball back into the urn, we add in another red ball. So the number of balls, and the number of red balls increases for this round. The probability of drawing a blue ball is now 3, so P X 3. P X > P X P X < } {{ } P X X 3 is the case that the first blue ball drawn is chosen on the third round. This means the first two draws were red which occurs with probably 3 and the third draw is the blue ball, which occurs with probability 4. Thus, P X P X > 3 P X 3 P X < 3 } {{ } P X+P X and so on... If you are bored, you can prove this using mathematical induction, but we will just accept it here. We note that, P X > i i i+ i+ b Show that with probability, a blue ball is eventually chosen. Show P X <. Proof. We can use the result from part a to instead prove that P X > 0. We take the limit as i. P X > lim i lim i P X < i + i + i 0, so

12 c Find EX. Recall that the definition of EX is EX n x i P X x i i In this context, EX i ip X i, but we need to find a representation for P X i. For X i, this means that the first i draws were all red, and the ith draw was blue. Then, Then, P X i 3... i } {{ i } } i + {{ } i red balls drawn blue ball drawn ii + EX x i P X x i i i ii + i + i i which diverges by the basic comparison test since i i diverges. EX This is a variation of the Polya urn model.

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