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  Statistical Techniques and 6 Sigma
  Stability tests

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Author Topic:   Stability tests
Dawn
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posted 26 September 1998 07:53 PM     Click Here to See the Profile for Dawn   Click Here to Email Dawn     Edit/Delete Message   Reply w/Quote
I need to know how to calculate control limits for stability tests. Any experts?
Thanks ahead of time !!

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Marc Smith
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posted 27 September 1998 01:20 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
I'm posting a reply to 'light the bulb' - maybe attract a reply. I'm not a statistical person. If you get real needy, e-mail Howard Atkins - he's the moderator of one of the forums. He's pretty statistical.

Anyone have an answer for here?

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Don Winton
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posted 28 September 1998 10:21 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Dawn,
I may be able to help, but need more info. Can you provide additional details of your requirement.

Best Regards,

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Dawn
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posted 28 September 1998 06:43 PM     Click Here to See the Profile for Dawn   Click Here to Email Dawn     Edit/Delete Message   Reply w/Quote
I am writing a procedure on claculating stability. When I read in the MSA book a calculation which I don't have with me right now, but I was confused as to how to claculate the control limits. We have started stability tests on a chart, and it is a regular Xbar chart. We need to calculate the control limits to get an accurate range of the readings. Does anyone know of SPC programs which will give you the actual stability of the master part?
Thanks for the help!

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Don Winton
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posted 29 September 1998 02:08 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Dawn,

You have started stability tests using a „regularš Xbar chart. However, you did not mention a chart for dispersion (R, s, sigma, etc.)? Assuming a dispersion charts exists, then the calculation of the limits is straightforward for a „regularš control chart. If a chart for dispersion does not exist, that is another matter.

Control limits do not give an accurate range of the readings. Rather, the purpose of control limits is to determine when a process is in or out of a state of statistical control. A process can be in control and still not be stable. Perhaps you are referring to process capability? It sounds, however, that what you are trying to do is not typical. Perhaps I am still missing something. Additional details may be required.

There are many SPC programs, but I prefer Statview with the QC module installed.

Best Regards,

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Marc Smith
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posted 29 September 1998 09:51 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
I used Statview in the somewhat distant past and liked the software.

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Dawn
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posted 03 October 1998 07:13 PM     Click Here to See the Profile for Dawn   Click Here to Email Dawn     Edit/Delete Message   Reply w/Quote
Thanks Don,
We have a customer who wants us to perform stability tests on every gage we use to machine their parts, including micrometers. We do not have an SPC program which calculates stability.I have discussed this with someone and was informed to use moving range charts and we should be ok with that. Haven't tried it yet. Thanks!!!

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Don Winton
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posted 04 October 1998 09:18 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Dawn,

Be sure to discriminate the difference between „stabilityš and „statistical process control.š I would suggest that your customer does not know the difference, either. Rather than stability tests, would not repeatability and reliability (R&R) tests be more suitable? Maybe, Maybe not.

Best Regards,

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Marc Smith
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posted 05 October 1998 10:04 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Enter MSA...

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Don Winton
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posted 13 October 1998 08:33 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Agreed, Marc. MSA needs to be applied here. SQC has gone as far as it can.

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Lyndon Diong
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posted 15 December 1998 01:48 AM     Click Here to See the Profile for Lyndon Diong   Click Here to Email Lyndon Diong     Edit/Delete Message   Reply w/Quote
Don,
Glossary in SPC ref manual says:
Stability - The absence of special causes of variation; the property of being in statistical control. What do you think? Thanks

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Don Winton
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posted 15 December 1998 11:46 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Lyndon, thanks for raising this question. It gives me the opportunity to correct some past wrongs. "Whoever undertakes to set himself up as a judge in the field of truth and knowledge is shipwrecked by the laughter of the gods." Albert Einstein

First, I must offer a very humble apology. My statement above: "A process can be in control and still not be stable" is not entirely correct. This goes back to my interpretation of SQC theory. "A process can be in control and still not be capable" is a more precise statement. While I agree (in part) with the definition supplied from the SPC reference manual you mentioned, there are still issues with the term "stability" I feel should raised.

While some prefer to use stability to refer to a process that is in a state of statistical control, it is not exactly precise to do so. When using SQC techniques, the ROOTS of the statistical theory should be remembered (all too often, the hammer gets forgotten in the process of driving a nail, that is, until it strikes your thumb).

We expect "almost all" points which represent samplings from a 'stable process' to fall inside 3-sigma limits. If they do not, we say "The process is not in control" and there is only a small risk of the conclusion being incorrect. If they do all fall inside, we say "The process appears to be in statistical control" or "The process appears stable"; which is not the same as saying "It IS stable." There is an analogy. A person is accused of a crime: The evidence may (1) convict the defendant of guilt, and we realize there is some small chance that justice miscarried, or (2) fail to convict, but this is not the same as believing in the person's innocence." From Process Quality Control, Ott and Schilling, 1990. This is, in a nutshell, a description of the null hypothesis.

IMHO, a more precise definition that should have been supplied in the SPC reference manual would be:

Stability: When a process appears to be in a state of statistical control and there is a lack of evidence of special cause variation.

I do not have, or have access to, the SPC manual mentioned. What is its source?

As you can probably tell by now, I prefer the term 'stability' not be used (EVER) to describe a process, or in this case, gage. Going back to Dawn's original post, all the stability tests ever conducted would not tell when a process is capable. The micrometers in question, for example. By 'fouling' the micrometer, I could present data set after data set that showed it to be very stable, but it would be 1" (made up number) OFF in accuracy. Would not R&R, MSA, process capability or Taguchi's Loss Function be a more suitable statement of the gage's fitness for use? When used correctly, SQC is a very powerful instrument. Of course, the inverse is also true.

Anyway, just my $0.02 worth.

Regards,
Don

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Dawn
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posted 15 December 1998 07:04 PM     Click Here to See the Profile for Dawn   Click Here to Email Dawn     Edit/Delete Message   Reply w/Quote
Correct me if I'm wrong, but we are confusing the stability of a process with the stability of a gage?

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Don Winton
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posted 15 December 1998 07:43 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Dawn,

Are they not the same? The theories are.

After all, a process creates a product. The stability of the gage produces a product. The product is the data in question.

Reference ISO 9000:

3.1 Product: Result of activities or processes.

From the notes: A product can be tangible (e.g., assemblies or processed material) or intangible (e.g., knowledge or concepts), or a combination thereof. Additional discussion is available from the additional notes of ISO 9000 and ISO 8402. I would think that the QS definition is similar. Maybe, maybe not.

I humbly suggest that whenever a gage is subjected to the őSo-Called‚ stability requirement, ask this question: „What are you looking for? Stability or accuracy?š

Reference my posts above (excluding the errors mentioned).

Regards,
Don

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Marc Smith
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posted 20 March 2000 12:05 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Also see https://elsmar.com/ubb/Forum4/HTML/000079.html

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