When the process is stable, the capability is monitored by the control limits and the index Cpk.

To look more at control limits, you can look the chapter 5.2, Discrete variable control chart, and chapter 5.2 – Continuous variable control chart.

The index CPk additional information on top of the control limits, like:

  • Monitor how close the process is to the specification limits;
  • A higher value of CPK means that you have more probability of hitting the mean of the process;
  • A negative CPk state that the process produces all the output that is out of the specification limits;
  • Generally, you want a CPk of at least 1.33, which is four sigmas (*).

(*) If you want to pass from the CPk to the sigma level of your process, you can multiply the CPk * 3

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