The financial manager of a large department store chain selected a random sample of 200 of its credit card customers and found that 136 had incurred an interest charge during the previous year because of an unpaid balance.

a. Compute a 90% CI for the true proportion of credit card customers who incurred an interest charge during the previous year.

b. If the desired width of the 90% interval is .05, what sample size is necessary to ensure this?

c. Does the upper limit of the interval in part

(a) specify a 90% upper confidence bound for the proportion being estimated? Explain.