In tough economic times, the business staff at magazines are challenged to sell advertising space in their publications. Thus, one indicator of a weak economy is the decline in the number of "ad pages" that magazines have sold. The file Ad Pages contains the number of ad pages found in the May 2008 and May 2009 issues of 12 men's magazines. (Data extracted from W. Levith, "Magazine Monitor," Mediaweek, April 20, 2009, p. 53.)
a. At the 0.05 level of significance, is there evidence that the mean number of ad pages was higher in May 2008 than in May 2009?
b. What assumption is necessary about the population distribution in order to perform this test?
c. Use a graphical method to evaluate the validity of the assumption in (b).
d. Construct and interpret a 95% confidence interval estimate of the difference in the mean number of ad pages in men's magazines between May 2008 and May 2009.