Why can process capability indices only be calculated reliably when the process is stable?

Name: Why can process capability indices only be calculated reliably when the process is stable?
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Quizplus · Accepted Answer

The Answer is When the process is stable, it is predictable. If the process is unpredictable (out-of-control) then we don't know what to expect in the future and the Cp and Cpk values are of little value.

Why Can Process Capability Indices Only Be Calculated Reliably When