How do you choose the best process capability index for your situation?

Cpk and Ppk do not measure how well a process is centred. It is the distance of the mean from both specification limits (assuming a bilateral specification) that are used in the calculation. Benchmark Cpk/Ppk results can be obtained from very ‘off centre’ processes provided the variation is minimal. This is not necessarily the optimal situation if the characteristic in question is one that affects product performance. The Cpk/Ppk indices are most suited to the question around defect % or ppm. The terms short and long term descriptions can mislead. The Cpk metric is weighted to the short term due to the fact the sigma estimate is calculated using ranges whereas Ppk is based on RSS standard deviation. Within or overall better describes this.

…..only as long as your process is normally distributed. As soon a you start seeing that shif, it goes out the window. Always take that into consideration??

Cp shows the potential of your your process ( what could it be if process was centered) while Cpk shows the off centerness and what it is ( short term)!

Cpk only shows off centred ness when compared to Cp. On its own it doesn’t tell you how well centred a process is. Used together they can be useful. Using simple graphics such as a histogram with specification limits can compliment these statistics well and should be encouraged as should viewing the stability of the process on a control chart.

"The process of choosing the best index is the most challenging part of your SPC, M phase, LSS, or P of a Continuous Improvement PDCA. Here's my approach: Speak stakeholders' language: What do they want to see? Keep within tolerance: What can we do excellently? Don't try to be a hero: What resources do we have? I am limited to 750 Characters here, but you can find my full article about how I answer this question, in my page.

It’s really important to remember what the intent of the customer request is - clarify if you don’t think their selected metric is appropriate. It can be quite possible to give a distorted perception of the ‘true’ capability if your process behaves differently in short term than ‘long’ term. Say when larger variations exist between batches than within batches of product, or where gradual drifts occur naturally. In cases such as this Cpk may give a false impression of performance. Also be sure to clarify the methods and formulae the customer wants using. I’ve seen some businesses using Ppk but calling it Cpk. Refer to recognised standards such as AS13006 (AESQ aerospace standard) for standard formulas used in that industry.