# Solutions to Homework 6 Statistics 302 Professor Larget

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2 5.36 (Graded for Accurateness) Commuting TImes in St. Louis A bootstrap distribution of mean commute times (in minutes) based on a sample of 500 St. Louis workers stored in CommuteStLouis is shown in the book. The pattern in this dot plot is reasonably bell-shaped so we use a normal curve to model this distribution of bootstrap means. The mean for this distribution is minutes and the standard deviation is 0.65 minutes. Based on this normal distribution, what proportion of bootstrap means should be in each of the following regions? (a) More than 23 mintues (b) Less than 20 minutes (c) Between 21.5 and 22.5 minutes The plots below show the three required regions as areas in a N(21.97, 0.65) distribution. We see that the areas are , , and , respectively. If converting to a standard normal, the relevant z-scores and areas are shown below. (a) z = = The area above for N(0, 1) is (b) z = = The area below for N(0, 1) is (c) z = = and z = = The area between and for N(0, 1) is

3 5.62 (Graded for Accurateness) To Study Effectively, Test Yourself! Cognitive science consistently shows that one of the most effective studying tools is to self-test. A recent study reinforced this finding. In the study, 118 college student studied 48 pairs of Swahili and English words. All students had an initial study time and then three blocks of practice time. During the practice time, half the students studied the words by reading them side by side, while the other half gave themselves quizzes in which they were shown one word and had to recall it partner. Students were randomly assigned to the two groups, and total practice times was the same for both groups. On the final test one week later, the proportion of items correctly recalled was 15% for the reading-study group and 42% for the self-quiz group. The standard error for the difference in proportions is about Test whether giving self-quizzes is more effective and show all details of the test. The sample size is large enough to use the normal distribution. The relevant hypotheses are H 0 : p Q = p R vs H a : p Q > p R, where p Q and p R are the proportions of words recalled correctly after quiz studying or studying by reading alone, respectively. Based on the sample information the statistic of interest is ˆp Q ˆp R = = 0.27 The standard error of this statistic is given as SE = 0.07 and the null hypothesis is that the difference in the proportions for the two group is zero. We compute the standardized test statistic with SampleStatistic NullP arameter z = = = 3.86 SE 0.07 Using technology, the area under a N(0, 1) curve beyond z = 3.86 is only This very small p-value provides very strong evidence that the proportion of words recalled using self-quizzes is more than the proportion recalled with reading study alone (Graded for Completeness) Penalty Shots in World Cup Soccer A study of 138 penalty shots in World Cup Finals games between 1982 and 1994 found that the goalkeeper correctly guessed the direction of the kick only 41% of the time. The article notes that this is slightly worse than random chance. We use these data as a sample of all World Cup penalty shots ever. Test at a 5% significance level to see whether there is evidence that the percent guessed correctly is less than 50%. The sample size is large enough to use the normal distribution. The standard error from a randomization distribution under the null hypothesis is SE= We test H 0 : p = 0.5 vs H a : p < 0.5 where p is the proportion of all World Cup penalty shots for which the goalkeeper guesses the correct direction. The statistic from the original sample is ˆp = 0.41 and the null parameter is p = 0.5. The standard error is SE = We use this information to find the standardized test statistic: Samplestatistic Nullparameter z = = = This is a lower tail test, so we find the area below z = 2.09 in the lower tail of the standard normal distribution. We see in the figure below that this area is The p-value for this test 3 SE

4 is At a 5% significance level, we find evidence that the proportion of World Cup penalty shots in which the goalkeeper guesses correctly is significantly less than half (Graded for Completeness) Malevolent Uniforms in Football The figure in the book shows a bootstrap distribution of correlations between penally yards and uniform malevolence using the data on 28 NFL teams in MalevolentUniformsNFL. We wee from the percentiles of the bootstrap distribution that a 99% confidence interval for the correlation is to The correlation between the two variables for the original sample is r = (a) Use the original sample correlation and the standard deviation of the bootstrap distribution shown in the figure to compute a 99% confidence interval for the correlation using z from a normal distribution. (b) Why is the normal-based interval somewhat different front he percentile interval? Hint: Look at the shape of the bootstrap distribution. (a) For a 99% confidence interval the standard normal value leaving 0.5% in each tail is z = From the bootstrap distribution we estimate the standard error of the correlations to be Thus the 99% confidence interval based on a normal distribution would be 0.37 ± = 0.37 ± = ( 0.158, 0.898) (b) The bootstrap distribution of correlations is somewhat right skewed, while the normal-based interval assumes the distribution is symmetric and bell-shaped. Disjoint, Independent, and Complement For Exercise 11.30, state whether the two events (A and B) described are disjoint, independent, and/or complements. (It is possible that the two events fall into more than one of the three categories, or none of them.) (Graded for Accurateness) Roll two (six-sided) dice. Let A be the event that the first die is a 3 and B be the event that the sum of the two dice is 8. The two events are not disjoint or complements, as it is possible to have the rolls be {3, 5} where the first die is a 3 and the sum is 8. To check independence we need to find P (A) = 1/6 and P (B) = P ({2, 6} or {3, 5} or {4, 4} or {5, 3} or {6, 3}) = 5/36 4

5 There is only one possibility for the intersection so P (A and B) = P ({3, 5}) = 1/36. We then check that P (A and B) P (A if B) = = 1/36 = 1/6 P (B) = 5/36 P (B) 1/6 so A and B are not independent. We can also verify that P (A and B) = 1/36 P (A) P (B) = (1/6) (5/36) = 5/ (Graded for Accurateness) Peanut M & Ms In a bag of peanut M & M s, there are 80 M & Ms with 11 red ones, 12 orange ones, 20 blue ones, 11 green ones, 18 yellow ones, and 8 brown ones. They are mixed up so that each candy piece is equally likely to be selected if we pick one. (a) If we select one at random, what is the probability that it is red? (b) If we select one at random, what is the probability that it is not blue? (c) If we select one at random, what is the probability that is is red or orange? (d) If we select one at random, then put it back, mix them up well (so the selections are independent) and select another one, what is the probability that both the first and second ones are blue? (e) If we select one, keep it, and then select a second one, what is the probability that the first one is red and the second one is green. (a) There are 11 red ones out of a total of 80, so the probability that we pick a red one is 11/80 = (b) The probability that it is blue is 20/80 = 0.25 so the probability that it is not blue is = (c) The single piece can be red or orange, but not both, so these are disjoint events. The probability the randomly selected candy is red or orange is 11/ /80 = 23/80 = (d) The probability that the first one is blue is 20/80 = When we put it back and mix them up, the probability that the next one is blue is also By the multiplication rule, since the two selections are independent, the probability both selections are blue is = (e) The probability that the first one is red is 11/80. Once that one is taken (since we don t put it back and we eat it instead), there are only 79 pieces left and 11 of those are green. By the multiplication rule, the probability of a red then a green is (11/80) (11/79) = (Graded for Completeness) Color Blindness in Men and Women The most common form of color blindness is an inability to distinguish red from green. However, this particular form of color blindness is much more common in men than in women (this is because the genes corresponding to the red and green receptors are located on the X-chromosomes). Approximately 7% of American men and 0.4% of American women are red-green color-blind. (a) If an American male is selected at random, what is the probability that he is red-green colorblind? (b) If an American female is selected at random, what is the probability that she is NOT red-green color-blind? (c) If one man and one woman are selected at random, what is the probability that neither are 5

6 red-green color-blind? (d) If one man and one woman are selected at random, what is the probability that at least one of them is red-green color-blind? Let CBM and CBW denote the events that a man or a woman is colorblind, respectively. (a) As 7% of men are colorblind, P(CBM) = (b) As 0.4% of women are colorblind, P(not CBW) = 1 - P(CBW) = = (c) The probability the woman is not colorblind is 0.996, and the probability that the man is not color- blind is = As the man and woman are selected independently, we can multiply their probabilities: P(Neither is Colorblind) = P(not CBM) P(not CBW) = = (d) The event that At least one is colorblind is the complement of part (d) that Neither is Colorblind so we have P (At least one is Colorblind) = 1 - P (Neither is Colorblind) = = We could also do this part as P(CBM or CBW) = P(CBM)+P(CBW)-P(CBM and CBW) = (0.07)(0.004) = (Graded for Accurateness) Mammograms and Breast Cancer The mammogram is helpful for detecting breast cancer in its early stages. However, it is an imperfect diagnostic tool. According to one study, 86.6 of every 1000 women between the ages of 50 and 59 that do not have cancer are wrongly diagnosed (a false positive), while 1.1 of every 1000 women between the ages of 50 and 59 that do have cancer are not diagnosed (a false positive). One in 38 women between the ages of 50 and 59 will develop breast cancer. If a woman between the ages of 50 and 59 has a positive mammogram, what is the probability that she will have breast cancer? We are given Applying Bayes rule we have P (Positive if no Cancer) = 86.6/1000 = , P (Positive if Cancer) = 1 1.1/1000 = , and P (Cancer) = 1/38 = P (Cancer if Positive) = P (Cancer)P (Positive if Cancer) P (no Cancer)P (Positive if no Cancer) + P (Cancer)P (Positive if Cancer) = (0.0263)(0.9989) ( )(0.0866) + (0.0263)(0.9989) = Identifying Spam Text Messages Bayes rule can be used to identify and filter spam s and text messages. Exercise refers to a large collation of real SMS text messages from participating cellphone users. In this collection, 747 of the 5574 total messages (13.40%) are identified as spam. 6

7 11.60 (Graded for Completeness) The word free is contained in 4.75% of all messages, and 3.57% of all messages both contain the word free and are marked as spam. (a) What is the probability that a message contains the word free, given that it is spam? (b) What is the probability that a message is spam given that it contains the word free? (a) Using the formula for conditional probability, P (Free if Spam) = P (Free and Spam) P (Spam) = = (b) Using the formula for conditional probability, P (Spam if Free) = P (Free and Spam) P (Free) = = Computer Exercises For each R problem, turn in answers to questions with the written portion of the homework. Send the R code for the problem to Katherine Goode. The answers to questions in the written part should be well written, clear, and organized. The R code should be commented and well formatted. R problem 1 (Graded for Accurateness) The function pnorm() finds probabilities from normal distributions. By default, it returns the area to the left from the standard normal density, but the second and third arguments can be used to specify a different mean or standard deviation. So, here are various ways to calculate the area to the right of 650 from a N(500, 100) distribution. 1 - pnorm(650, 500, 100) ## [1] pnorm(650, mean = 500, sd = 100) ## [1] pnorm(( )/100) ## [1] The function qnorm() finds quantiles from a normal distribution. Again, without other arguments, it uses the standard normal distribution. qnorm(0.9) ## [1] qnorm(0.9, mean = 500, sd = 100) ## [1] Write an expression using pnorm() and or qnorm() to find each of the following values. Note, we are using the notation N(µ, σ) to represent a normal distribution with parameters µ for the mean and σ for the standard deviation (and not using N(µ, σ 2 )). For all problems, use the N(250, 30) distribution. 1. P (X < 200). 7

8 Using R, we determine that P (X < 200) = pnorm(200,250,30) 2. P (X > 260). Using R, we determine that P (X > 260) = 1 P (X < 260) = pnorm(260,250,30) 3. P ( X 250 > 40). Using R, we find that P ( X 250 > 40) = P ((X 250 < 40) (40 < X 250)) = P (X 250 < 40) + P (40 < X 250) = P (X < ) + P ( < X) = P (X < 210) + P (290 < X) = P (X < 210) + (1 P (290 < X)) = (1-pnorm(290,250,30))+(pnorm(210,250,30)) 4. P (260 < X < 300). Using R, we find that pnorm(300,250,30)-pnorm(260,250,30) 5. The number c so that P (X < c) = 0.9. P (260 < X < 300) = P (X < 300) P (X < 260) = Using R, we find that the value c such that P (X < c) = 0.9 is qnorm(0.9,250,30) 8

9 6. The number c so that P (X > c) = Using R, we find that qnorm(0.76,250,30) P (X > c) = P (X > c) = P (X < c) = The number c so that P ( X 250 > c) = Using R, we find that qnorm(0.91,250,30) c = P ( X 250 > c) = 0.18 P ((X 250 < c) (c < X 250)) = 0.18 P (X 250 < c) + P (c < X 250)) = 0.18 P (X 250 < c) + P (X 250 < c) = The number c so that P ( X 250 < c) = 0.9. First consider that 2P (c < X 250)) = 0.18 P (c < X)) = 0.09 P (c > X)) = P (c > X)) = 0.91 c = c = P ( c < X 250 < c) = 0.9 P (X 250 < c) P (X 250 > c) = 0.9 P (X 250 < c) (1 P (X 250 < c)) = 0.9 P (X 250 < c) 1 + P (X 250 < c) = 0.9 2P (X 250 < c) 1 = 0.9 2P (X 250 < c) = 1.9 P (X 250 < c) = P (X < c + 250) =

10 From R, we determine that c = c = qnorm(0.95,250,30) R problem 2 (Graded for Accurateness) The height of the density function of a normal curve can be computed with the R function dnorm(). Write a function called gnorm() that will draw a sketch of a normal density and shade in the two tails with probability α/2 if one passes in a mean, sd, and alpha value. Modify this function which calculates and draws P (X a). gnorm = function(a,mu=0,sigma=1) { # create an array of x values of length 501 # from 4 SDs below to 4 SDs above the mean x = seq(mu-4*sigma,mu+4*sigma,length=501) # calculate the height of the normal density at these points y = dnorm(x,mean=mu,sd=sigma) # put x and y into a data frame d = data.frame(x,y) # create a plot that graphs the normal density # and overlays this with a horizontal line for the x axis # store the plot as the object p and later add more to it require(ggplot2) p = ggplot(d, aes(x=x,y=y)) + geom_line() + geom_segment(aes(x=mu-4*sigma,xend=mu+4*sigma,y=0,yend=0), data=data.frame(mu,sigma)) + ylab( density ) # shade in the area # add points to the data frame d that are the bottom of the segment to shade # and extract only those x and y from d where x <= a d2 = data.frame(x = c(mu-4*sigma,x[x<=a],a), y = c(0,y[x<=a],0)) p = p + geom_polygon(aes(x=x,y=y),data=d2,fill="red") + ggtitle(paste("p(x <",a,") =",round(pnorm(a,mu,sigma),4))) plot(p) } Below is code for a function that will perform the desired task. anorm = function(alpha,mu=0,sigma=1) { x = seq(mu-4*sigma,mu+4*sigma,length=501) y = dnorm(x,mean=mu,sd=sigma) d = data.frame(x,y) require(ggplot2) p = ggplot(d, aes(x=x,y=y)) + geom_line() + geom_segment(aes(x=mu-4*sigma,xend=mu+4*sigma,y=0,yend=0), data=data.frame(mu,sigma)) + ylab( density ) 10

11 } a <- qnorm(alpha/2,mu,sigma) d2 = data.frame(x = c(mu-4*sigma,x[x<=a],a), y = c(0,y[x<=a],0)) b <- qnorm(1-alpha/2,mu,sigma) d3 = data.frame(x = c(b,x[x>=b],mu+4*sigma), y = c(0,y[x>=b],0)) p = p + geom_polygon(aes(x=x,y=y),data=d2,fill="red") + geom_polygon(aes(x=x,y=y),data=d3,fill="red")+ ggtitle(paste("p( X >",round(b,4),") =",alpha)) plot(p) For example, this code can be used in the following manner to create the image below. gnorm(.025,0,1) 0.4 P( X > ) = density x 11

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