Price scanners are widely used in U.S. supermarkets.

While they are fast and easy to use, they also make mistakes. Over the years, various consumer advocacy groups have complained that scanners routinely gouge the customer by overcharging. A recent Federal Trade Commission study found that supermarket scanners erred 3.47% of the time and department store scanners erred 9.15% of thc time ("Scan Errors Help Public,"

Newark Star-Ledger, Oct. 23,1996).

a. Assume the above error rates were determined from merchandise samples of size 800 and 900, respectively. Are these sample sizes large enough to apply the methods of this section to estimate the difference in the error rates? Justify your answer.

b. Use a 98% confidence interval to estimate the difference in the error rates. Interpret your result.

c. What assumptions must hold to ensure the validity of the confidence interval of part b?