First Stability then Capability Analysis
I recently got a question from a QI Macros user. His Cp/Cpk was high, while his Pp/Ppk were low. He wondered if there was a problem with the calculation in the QI Macros.
Whenever Cp is more than a third higher than Pp, it means the process isn’t stable. And a process has to be stable before you can do capability analysis. How do you check stability? With a Control Chart. Here’s the data in an XmR chart:
As you can see, the process runs at three different levels. I have found that any process shift, not matter how small will create out of control conditions that prohibit capability analysis. Pp and Ppk use standard deviation (distance from the mean). Cp and Cpk use the difference between parts to estimate standard deviation. When part-to-part variation is small, Cp/Cpk will get large.
Pp = (USL-LSL)/STDEV
Cp = (USL-LSL)/Sigma Estimator
The QI Macros will alert you to these issues, so you will need to go back and run a control chart to determine what’s going on. Then do some root cause analysis and correction to make the process stable.
In this case, the measurements need to go out to the 0.0001 decimal place to capture enough part to part variation to allow capability analysis.
Get the idea: First Stability then Capability