The accounts receivable department at Rick Wing Manufacturing has been having difficulty getting customers to pay the full amount of thier bills. Many customers complain that the bills are not correct and do not reflect the materials that arrived at thier receiving docks. The department has decided to implement SPC in its billing process. To set up control charts, 10 samples of 50 bills each were taken over a month's time and the items on the bills checked off against the bill of lading sent by company's shipping department to determine the number of bills that were not correct. The results were:
sample # # of incorrect bills
1 6
2 5
3 11
4 4
5 0
6 5
7 3
8 4
9 7
10 2
a) Determine the value of the p-bar, the mean fraction defective. Then determine the control limits for the p-chart using 99.73% confidence level (3 standard deviations). Is this process in control? If not, which samples were out of control?
b) How might you use quality tools to determine the source of the billing defects and where might you start your improvement efforts to eliminate the causes?