S6.25 The accounts receivable department at Rick Wing lanufacturing has been having difficulty getting customers to pay ef full amount of their bills. Many customers complain that the iils are not correct and do not reflect the materials that arrived at heir receiving docks. The department has decided to implement PC in its billing process. To set up control charts, 10 samples of 50 ills each were taken over a month's time and the items on the bills hecked against the bill of lading sent by the company's shipping lepartment to determine the number of bills that were not correct. The results were: Determine the value of p-bar, the mean fraction defective. Then determine the control limits for the p-chart using a 99.73% confidence level ( 3 standard deviations). Is this process in control? If not, which sample(s) were out of control? b) How might you use the quality tools discussed in Chapter 6 to determine the source of the billing defects and where you might start your improvement efforts to eliminate the causes