# Sigma Level Formula Dissertation Help Needed

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• #54264

Luís
Participant

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 = ?

0
#194504

Eric Maass
Participant

Hello, Luis,

I have never used that formula, preferring to use Excel’s built-in functions for cumulative normal distributions…but this is referred to as:
Schmidt S. and Launsby, Quality Progress, 1997

Best regards,
Eric

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#194506

Luís
Participant

Hello 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

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#194512

Eric Maass
Participant

Hi Luis,

Well, I checked out the articles from Quality Progress in 1997
( http://asq.org/qualityprogress/past-issues/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:

http://www.amazon.com/Basic-Statistics-Continuous-Improvement-Edition/dp/1880156067/ref=sr_1_3?ie=UTF8&qid=1356869114&sr=8-3&keywords=basic+statistics+schmidt

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,
Eric

0
#194513

Eric Maass
Participant

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/Understanding-Industrial-Designed-Experiments-4th/dp/1880156032

So, 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: