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March 2007

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Subject:
From:
Lee parker <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Lee parker <[log in to unmask]>
Date:
Wed, 28 Mar 2007 11:45:53 -0400
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Ryan

Put Cpk into your search engine and you should get many references.

I imagine the reason for this low Cpk was that the standard deviation was 
large even though the average was with in range. This indicates a poorly 
controlled process that has an unacceptable likelihood of producing 
unacceptable product, which is the purpose of the measurement.

Best regards

Lee

J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389


----- Original Message ----- 
From: "Ryan Grant" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, March 28, 2007 11:39 AM
Subject: [TN] Explanation of Cpk (Ppk)


Hi all,

I have a supplier that is wondering why we rejected a shipment for
finding a critical dimension as having a Cpk of 0.65 (Ok, technically,
we are calculating Ppk).  None of the 45 measurements were actually out
of spec (although one measurement just missed exceeding the upper limit
by 0.002mm).  Now they want to know why we are rejecting for low Cpk.
Sigh...

Rather than go through the tedium of carefully explaining what a
distribution is, what it means, how to calculate it and what the
calculation implies, etc., does anyone know of some websites I can point
them to that can explain this?

Any help is appreciated.

Thanks,
Ryan Grant

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