I am not familiar with Minitab either. I also do not have a copy of the AIAG manual. But, as I elaborated on in the forwarding thread I referenced above, I believe AIAG has produced more problems by creating this Ppk thing. I had never heard of it before the forwarding thread above, nor had I heard of short-term or long-term capability studies.

All process capability studies should use an unbiased estimate of standard deviation or calculated standard deviation as detailed in the CPK.PDF file located at the PDF Zone section of the Cove. It states:

"To obtain an accurate estimate of the process spread (standard deviation), at least sixty data points are needed. If less that sixty data points are available, use the following formula with error correction factors given in Table 1."

Basically, if there are less than sixty data points, you use an unbiased estimate of standard deviation corrected by the formula. If greater than sixty data are available, use the standard deviation calculated from the data.

BTW, which symbol (s, sigma-hat, s-hat, etc.) does the AIAG manual use in the formula for Ppk and Cpk to represent process spread?

Regards,

Don

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Just the ramblings of an Old Wizard Warrior.