12.29 Alloy composition—prospective control.
Project the and chart limits found in the previous exercise for prospective control of aluminum content. The last 15 rows of Table 12.7 give data on the next 15 future subgroups. Refer to Exercise 12.27, and apply the nine-in-a-row rule along with the standard three-sigma rule to the new subgroups. Is the process maintaining control? If not, describe the nature of the process change and indicate the subgroups affected.
alloy
12.29
Although the data points are all within the control limits, starting with subgroup 27, there are 9 data points below the center line, violating the nine-in-a-row rule. It is likely the process level has shifted and/or there is special cause acting on the process that needs to be determined.