Cpk and other capability indices should be calculated for stable processes only. This is a critical requirement for calculating capability indices. Sadly, it is the most ignored fact in the real world.
What is a stable process?
A process that does not have any special causes present is a stable process. Such a process has inherent variation due to common causes only. The control chart for a stable process has no 'Out of control' points.
Why is stability important for Capability studies?
The capability indices are designed to determine how capable an ongoing process is for consistently delivering product within customer specifications given the natural variation of the process.
Ongoing processes should typically have only common cause variation i.e. the process is statistically stable.
When assignable causes come into play, the process becomes unstable. Now the control chart has out of control points that need to be watched closely and removed as soon as possible.
When processes are unstable, it is recommended to use performance indices like Pp and Ppk.
For more information on capability indices, please read our White Paper Use of different capability indices.
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