Excellent points Grant but why would we use Cpk to define stability if, as you stated, it is independent of customer requirements?
A process is in control if the measure tracked forms a stable distribution over time. A simple control chart serves this purpose. Customer requirements are not needed to make a determination of process…[Read more]
Several of the responses on this subject seem to be confusing stability & capability.
Stability is simply the absence of assignable cause variation (including conformance with the Western Electric rules established by Walter Shewart). It is the voice of the process which is entirely independent of specifications.