You should create a range or sigma chart. These charts let you check for piece-to-piece variability.
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Are there points that are out of control on the range or sigma chart?
Yes No
A significant amount of the system’s output falls outside the specification limits. Look for ways to improve the process by reducing common cause variation. You may also need to evaluate the specification limits to determine if they are current.
A significant amount of the system’s output is within specification limits. This process is stable and is capable of meeting process requirements. Continue monitoring the process, since variation and process shifts can occur at any time.
The process is not centered on the target value. It may be producing within specifications; however, little, if any, output will “hit” the target value.
Is the Cpk greater than 1.0?
You do not have enough data to get good estimates of process spread and location. For a more accurate picture of your process, you should have at least 100 data values or 20-25 samples.
The process is centered on the target value. Is the Cpk greater than 1.0?
Is the Cpk equal to the Cp?
The process has “high” capability. Is the Cpk equal to the Cp?