QeSmithy said:

When evaluating process capability…

- A short term study is conducted to see how good a process can be. Data is collected over a short period of time during which the process is influenced only by random causes.Cp, Cpk

- For a Six Sigma process, Cp = 2.0 and Cpk = 1.5

I Hope this helps!

Smithy

Welcome, Smithy.

I thought a Cpk of > 2 was required for a "six sigma" process. Can you double-check?

Here is another opinion on the debate, FWIW, from an article I have by Dr. Mehernosh Kapadia:

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In 1991, ASQ / AIAG task force published the "Statistical Process Control" reference manual, which presented the calculations for capability indices ( Cp, Cpk ) as well as process performance indices ( Pp, Ppk ).

The difference between the two indices is the way the process standard deviation ( s ) is calculated.

Cpk uses s which is estimated using ( R-Bar / d2 ) or ( S-Bar / C2 ) .

Ppk uses the calculated standard deviation from individual data where s is calculated by the formula :

So the next question is which metric is best to report Cpk or Ppk ?

In other words, which standard deviation to use - estimated or calculated ?

Although both indices show similar information, they have slightly different uses.

Ppk attempts to answer the question "does my current production sample meet specification ?" Process performance indices should only be used when statistical control cannot be evaluated.

On the other hand, Cpk attempts to answer the question "does my process in the long run meet specification?" Process capability evaluation can only be done after the process is brought into statistical control. The reason is simple: Cpk is a prediction, and one can only predict something that is stable.

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Dr. Mehernosh Kapadia is General Manager - Quality Engineering, Supply Chain Management Business Group at Tata AutoComp Systems Limited.

He has a Master's Degree in Mechanical Engineering from Penn State University, USA and MBA from NTU, USA. He is a Six-Sigma Master Black Belt and an ASQ certified CQE & CRE. He obtained his Doctorate from The University of Bombay