Actually, my calculations indicate one of the 45 measurements should
have been out, and one very nearly was. (The mean is shifted from
center, so unless you knew the amount of shift, assuming a centered
process, more than 2 measurements should have been out of spec.)
Actually, I have the fortunate benefit of being able to compare
measurements of that specific lot from two different receiving factories
around the world. The tails are still there, but slightly cut off of
the high end. The spread is still pretty wide. They may have screened
and sorted the product, but until I know that, I can only assume the out
of spec product is in there somewhere...(and I don't want to be the one
looking for it!)
-Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Louis Hart
Sent: Wednesday, March 28, 2007 10:20 AM
To: [log in to unmask]
Subject: Re: [TN] Explanation of Cpk (Ppk)
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
[log in to unmask]
Ryan Grant <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
03/28/2007 11:39 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
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To
[log in to unmask]
cc
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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