Ryan, to my eye, it is curious that of 45 measurements, with a Cpk of 
0.65, none were out of spec. My quick and perhaps erroneous calculation 
suggests you could expect 2 or more of the 45 measurements to be out of 
spec. It looks to me like the supplier's process is not so good, and he is 
doing a 100% screen of product before shipping it to you. The distribution 
of data may be like that of a normal, with the tails truncated.

Lou 

Louis Hart
Technology Transfer Specialist
National Technology Transfer Center
316 Washington Av
Wheeling, WV 26003
304-243-2553
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Ryan Grant <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
03/28/2007 11:39 AM
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[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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