Wal-Mart buyers seek to purchase adequate supplies of various brands of toothpaste to meet the ongoing demands of their customers. In particular, Wal-Mart is interested in knowing the proportion of its customers who favor such leading brands of toothpaste as Aquafresh, Colgate, Crest, and Mentadent. The file P8_41.XLS contains the toothpaste brand preferences of 2000 Wal-Mart customers, obtained recently through the administration of a customer survey. 

a. Determine the proportion of Wal-Mart customers who favor each major brand of toothpaste.

b. Assuming that the given data constitute an appropriate frame, choose a simple random sample of 100 of these customers.

c. Calculate the proportion of Wal-Mart customers in the random sample who favor each major brand of toothpaste. Compare these sample proportions to the corresponding values found in part a. How do you explain any disparities between corresponding proportions for customers included in the sample and those in the frame?