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  SPC and GD&T (Page 2)

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Author Topic:   SPC and GD&T
Roger Eastin
Forum Wizard
posted 06 August 1999 07:14 AM     Click Here to See the Profile for Roger Eastin   Click Here to Email Roger Eastin     Edit/Delete Message
There's actually been a LOT of material written in the Cove about the topic of the application of Cpk's to processes - normal and non-normal. I believe, in a nutshell, there are two broad areas to investigate: the use of PPM(non-parametric approach) or using an approximation to Cpk based on how "non-normal" your distribution is. Regarding the second approach, your Cpk can be fairly accurate even if the distribution is non-normal. The robustness of your Cpk depends on how "robust" the standard deviation is and that depends, in part, on the type of distribution your process follows. A good SPC book (Grant and Leavenworth, Wheeler, etc) would help. Generally, though, the more "tailed" your distribution is, the more problems are created for Cpk calculations. You should search the old forum for Cpk discussions because I'm sure that this has been covered before. Also, a while back, Bert Gunter put together an excellent series of articles in Quality Progress (when he was still writing for the "Statistics Corner") on Cpk applications. Try calling the ASQ and seeing if they can give you a transcript or, if I still have them, I can fax them to you.

Don Winton
Forum Wizard
posted 06 August 1999 09:00 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message
Good answer, Roger, thanks.

Tim,

Try these for now and I will have more over the weekend.

Regards,
Don

https://elsmar.com/ubb/Forum10/HTML/000025.html
https://elsmar.com/ubb/Forum10/HTML/000024.html
https://elsmar.com/ubb/Forum10/HTML/000015.html

Don Winton
Forum Wizard
posted 06 August 1999 09:23 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message
quote:
"Interesting concept but there is no way this can provide a normal distribution."

What I find interesting is that someone would say this without either data nor prior experience to support this statement. I usually view this as a Īwarningā sign that either the personās mind is already made up or they are not trained in statistical techniques.

Either way, I would view this comment as suspect. As Roger well stated, it is not necessarily normality that matters, but how strictly you wish to follow the Īrules.ā Those that have read my posts know I have bent and broken more that I have followed, with somewhat of success (although I view myself as rather less that Īexpertā).

BTW, the test for normality is a Skewness test. If the data are not normal, there are ways to Īconvertā it to normal. Too detailed to go into here, but any good statistical test will give the procedure.

The contractor has been negotiating with a separate consultant who has told them that Cpk will not work in conjunction with GD&T.

Wrong! Wrong! Find another consultant (would I do )

As far as I can tell from all of my research, the distribution must be normal for Cpk to mean anything.

No, it must not. It should be, but as I stated above, the normality depends upon how far you are willing to stretch the rules. And, if you are getting Cpk from a control chart (which you could do in the case of MMC), the law of subgroups would apply. This states that regardless of the population distribution, the subgroup sample data would reasonably simulate a normal (see the links above).

The one fixed rule for Cpk is that the process must be in a state of statistical control.

My advice: ditch this consultant and find one trained in advanced statistical techniques, or one that has at least read Grant and Leavenworth.

Ask your questions here. I will help as best I can. If your data are available, e-mail them and I will also help as best I can.

Good luck.

Regards,
Don

Batman
Contributor
posted 08 August 1999 10:38 AM     Click Here to See the Profile for Batman   Click Here to Email Batman     Edit/Delete Message
May I heartily agree with Don, throw out that consultant.

As I recall, the MMC Cpk paper is how to calculate Cpk based on the allowable bonuses. How can that restrict a process from being normally distributed or not?

Don Winton
Forum Wizard
posted 08 August 1999 08:34 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message

How can that restrict a process from being normally distributed or not?

I thank you Batman for your support. I had thought my ramblings had been of a negative nature, which had been the intent. I was absolutely upset that someone would presume this much.

Any process, regardless of the origin and/or source, will either become normal over a continuous period of time or the source of non-normality will sought out and corrected. For anyone to presume that this is not so is either non-enlightened or cares not to be.

BTW, check out the Wizards Way here:*** Dead Link Removed ***

Just some stuff I put together that has nothing to do with this forum.

Regards,
Don

Just the Ramblings of and Old Wizard Warrior

Visit dWizard's Lair:*** Dead Link Removed ***

Tim
Lurker (<10 Posts)
posted 09 August 1999 07:16 AM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message
Don and Batman,

Thanks for the vote of confidence. I have given this a lot of thought over the weekend and come to the conclusion that this consultant has a method to sell. The paper I received and forwarded basically cuts him out of a contract. I believe his business half of the brain overrode his judgement. I'll get some data and do some tests.

Thanks Again,
Tim

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