Sigma Level Formula Dissertation Help Needed
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 This topic has 4 replies, 2 voices, and was last updated 8 years, 5 months ago by Eric Maass.

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December 26, 2012 at 10:53 am #54264
Hello everyone! I need help with this formula for my dissertation. Can anyone tell me what is the origin of this sigma level formula: 0.8406 + SQRT [29.37 2.221 * ln(PPM)]. Many thanks!
0,8406 = ?
29,37 = ?
2,221 = ?0December 26, 2012 at 4:20 pm #194504
Eric MaassParticipant@poetengineer Include @poetengineer in your post and this person will
be notified via email.Hello, Luis,
I have never used that formula, preferring to use Excel’s builtin functions for cumulative normal distributions…but this is referred to as:
Schmidt S. and Launsby, Quality Progress, 1997Best regards,
Eric0December 27, 2012 at 5:25 am #194506Hello Eric,
Thank you for the reply. However I’ve been unable to find that article, in spite of much searching.
I would appreciate if you have the kindness of sending it to me.
8
Best regards,Luís
0December 30, 2012 at 4:09 am #194512
Eric MaassParticipant@poetengineer Include @poetengineer in your post and this person will
be notified via email.Hi Luis,
Well, I checked out the articles from Quality Progress in 1997
( http://asq.org/qualityprogress/pastissues/index.html?fromYYYY=1997 ) and did not find the referenced article.So, continuing on as a nerdy Sherlock Holmes, I found this comment:
QUALITY DIGEST  SAT, 06/05/2004
Hi, I noticed that you reference Schmidt & Launsby 1997 on pg 189 of the above text. I believe the reference should be Schmidt et al’s Basic Stat’s book and not Understanding Industrial DOE?I think this is the Basic Statistics book that is mentioned:
Having said that…I have to admit, I still don’t understand why people would use an equation to approximately estimate a sigma level using square roots and natural logarithms, which a person would enter into Excel, when Excel already has functions that will make the calculation directly. I guess I must be missing something.
Anyway, have a very Happy New Year!
Best regards,
Eric0December 30, 2012 at 4:19 am #194513
Eric MaassParticipant@poetengineer Include @poetengineer in your post and this person will
be notified via email.I skipped a step in sharing the search…
Here is the reference that the anonymous comment said was in error:Schmidt, S.R. and R.G. Launsby, Understanding Industrial Designed Experiments, Air Academy Press, Colorado Springs, CO, 1997.
You could also get that from amazon.com:
http://www.amazon.com/UnderstandingIndustrialDesignedExperiments4th/dp/1880156032So, I guess you could also check out that book, and see if the comment about an error was itself in error.
Alternatively – since Schmidt apparently developed the formula for Air Academy, I guess you could contact Air Academy itself and ask for their help on this:
http://www.airacad.com/ 1 (800) 7481277
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