Two students, Miguel Melo and Cody Watson, compared textbook prices at the campus bookstore and Amazon.com. To be fair, they included the sales tax for the local store and added shipping for Amazon. Here are the prices for a sample of 27 books.

a. Determine a 95% confidence interval for the difference of population means using the t method. Even if the normality assumption is not valid here, explain why the t method might still be appropriate.

b. Based on the 27 differences, use software to obtain a bootstrap sample of mean differences. Check the bootstrap distribution for normality.

c. Use the standard deviation of the bootstrap distribution along with the mean and t critical value from (a) to get a 95% confidence interval for the difference of means.

d. Use the bootstrap sample and the percentile method to obtain a 95% confidence interval for the difference of means.

e. Compare your three confidence intervals. In the light of your results for (d), does nonnormality invalidate the results of (a) and (c)? Explain.

f. Interpret your results. Is there a substantial difference between the two ways to buy books? Assuming that the populations remain unchanged and you have just these two sources, where would you buy?

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Campus Amazon 100.41 106.94 99.34 113.94 51.53 61.44 20.45 31.59 29.89 83.94 107.74 115.99 28.69 70.66 98.81 111.56 Campus 59.50 87.66 26.56 44.63 96.69 18.06 103.06 14.61 Amazon 69.24 73.84 33.98 40.39 117.99 27.94 115.74 24.69