Hi all, Steve Ouellette, Six Sigma Heretic here.
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First off, I am glad to see Dr. Wheeler add to his earlier article. If he had included the second and third paragraphs in his first article, I would have had much less concern that our readers would have made the erroneous conclusion that I feared they would.
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Dr. Wheeler starts off with a strawman argument: that I am arguing with the math. The math is not the issue, what I was arguing against was changing rejection specifications without an understanding of your process. Following the that first article without knowing more than was in the article would have led to, yes, disaster.
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I will hazard a guess and say that Dr. Wheeler and I would agree that you should tighten the rejection specifications as shown in his original article if:
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-you have no clue about your process through time (control)
-you have a measurement system with large measurement error as compared to the specification AND an as-measured Cpk lower than 1
-you have a process where there is large liability associated with making an incorrect classification and a measurement system that varies by some "large" amount as compared to the specification OR the process is out of control
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My position though, is that if any of those pertain at best you use the tightened rejection limits for a very limited amount of time until you fix your process. If these situations do not pertain, you are in control and your as-measured Cpk (which includes measurement error) is greater than or equal to 1, tightening the rejection limits only costs you money for protection against an eventuality that is very unlikely to happen.
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Heck, even if you have a Cpk of less than 1, with a huge enough %R&R you probably are still not making anything out of spec. The problem is convincing anyone of that...and detecting a real process shift in time to stop it from shipping.
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In my experience, most processes today are not like the ones described above. Most industries no longer live in the world of Cpks of 0.5. And if you do, get out! :-)
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Oh, and the ICC used for gauge measurement *acceptability* is bogus. Plain and simple. It is the right answer to the wrong question: "Can my gauge detect out-of-control events on the control chart?" The right question for gauge acceptability is: "Can I use this gauge to properly make conformance decisions while controlling risk?" Which is properly informed by (not decided by) %R&R or P/T ratios. (My previous articles too talked about taking an average of readings in order to reduce measurement error - just be sure your measurement system is in control before doing so!)
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I think the whole thing hinges on understanding the difference between measurement system capability (comparing measurement error to the spec width, e.g. P/T or %R&R) and measurement system acceptability (can I use the gauge in this application to make conformance decisions), which uses capability as one input.
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Ironically, I don't think Dr. Wheeler and I disagree on actions to take most of the time. My concern was his first article as-written told people to do something that probably doesn't make sense for them to do, and didn't clarify when the procedure was useful. Hopefully my article provided a service in doing do.
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Oh, and on my "math errors" - loosening the spec in the presence of gauge error doesn't make any sense in any context, and isn't what the article described, so not sure what Dr. Wheeler is referring to here. Feel free to educate me. Guardbanding still isn't needed in the vast majority of processes I see anyway...

Steve apparently has the luxury of seeing lots of processes with Cpks greater than 0.5.; my experience is that "most" companies are still pretty far from having this universal. I have worked with many companies where they had no SPC going at all yet. Just getting them to the threshhold state (in control, some nonconformance) was a challenge.

In Wheelers column "Is the Part in Spec?", there is a section titled Watershed Specifications in which he describes how the manufacturing specifications are adjusted to account for discontinuity as a result of gage resolution. Im not sure I would go so far as to say Wheeler would consider this is equivalent to "widening" the specifications, but the effect is similar in that numerical value range of the Watershed Specifications is indeed larger than that of the Manufacturing Specifications.

I have read Wheelers manuscripts from his reading room and I must say there are some very thorough arguments on how traditional education of gage R&R makes a number of "leaps of faith" - Wheeler does a good job of filling those gaps with sound mathematics, presenting the intraclass correlation coefficient as the correct workhorse for characterizing the measurement system in context of the product being judged.

Side note: I think the QD community benefits when experts such as Wheeler and Oullette spar a bit... here's to both of you for stickin' your necks out for the benefit of the rest of us :)

Thank you Don. Both of your articles are very clear, useful and easy to understand.

By contrast, Ouellette’s responses are tainted by emotion “crazy”, “disastrous”, perhaps driven by tall poppy syndrome; are vague; and show a lack of understanding. At least Ouellette calls himself a six sigma "heretic", so I can assume he is not quite as deluded as the masses that blindly follow the six sigma nonsense.

Some of my objectives are:

The "Belt" mentality
The mythical 1.5 sigma process shift
Does not address the homogeneity question (predictable vs. unpredictable process)
Obsession with the Normal Distribution
Inflated $ money saved claims

Rich DeRoeck

Donald - you did a masterful job explaining this. I read both previous articles, and found this explanation very thorough and complete. Steve made a good point in his comment that your first article could have been misinterpreted by someone without the knowledge base you posses (I almost made the very error counseled against in the latest paper due to my lack of experience).
This article was both helpful and clear, without being overbearing. Thanks for taking time to respond to Steve's critique - it shows your willingness to listen, and shows in part why you have been as successful as you have. The lack of arrogance is refreshing, and the tone of the article was anything but confrontational or defensive; both would have been easy mistakes.
Props to both of you for expanding my knowledge base, and for taking time to explain the intricate thought processes that go into making a decision.

Doug