Problem:
The Commonwealth Banking Corporation issues a national credit card through its various bank branches in five southeastern states. The bank credit card business is highly competitive and interest rates do not vary substantially, so the company decided to attempt to retain its customers by improving customer service through a reduction in billing errors. The credit card division monitored its billing department process by taking daily samples of 200 customer bills for 30 days and checking their accuracy. The sample results are as follows:
Sample Number of Defectives Sample Number of Defectives
1 7 16 10
2 12 17 12
3 9 18 14
4 6 19 16
5 5 20 15
6 8 21 13
7 10 22 9
8 11 23 10
9 14 24 12
10 10 25 15
11 9 26 14
12 6 27 16
13 3 28 12
14 2 29 15
15 8 30 14
Develop a -chart for the billing process using control limits and indicate if the process is out of control.