Sigma level from process capability

According to the book: Basic Statistics Tools for Continuous Improvement, by  Kiemele, Schmidt, and Berdine (Air Academy Associates airacad.com). Pages 9-11-915.
 
Sigma level = number of std deviations between the center of the process and the nearest spec (this is the Z value);  
Where Z value = Minimum ( (USL-xbar)/sdev, ((xbar-LSL))/sdev))
 
Cpk = Sigma Level/3
 
Sigma Capability = 0.8406 + SQRoot(29.37- 2.221 + ln(dpm))
 
Sigma Capability ¡Ö Sigma Level + 1.5.
 
Sigma Capability vs dpm
(Motorola¡¯s sigma capability index and dpm)
 

Sigma Capability

dpm

6.0

3.4

5.0

2.33

4.0

6,210

3.0

66,807

2.0

308,538
 
I hope it helps.
 
Al

This table is completely incorrect. 
Do not use it !

The origins of six sigma’s numbers will help people understand how the many error’s have been created. A brief summary follows:

Bill Smith, a Motorola engineer claims that for uncontrolled processes “batch to batch variation can be as much as +/-1.5 sigma off target.” He gives no references or justification for this. In reality there is no limit to how much that uncontrolled processes may vary.
At that time Motorola used Cp=1. Bill Smith suggested “Another way to improve yield is to increase the design specification width.” He broadens specification limits to Cp=2.
Mikel Harry derives +/-1.5 as a theoretical “shift” in the process mean, based on tolerances in stacks of disks. Stacks of disks obviously bear no relation to process.
Harry names his Z shift. The Z shift makes a number of additional errors: his derivation dispenses with time yet he refers to time periods; he claims a “short term” and “long term” yet data for both are taken over the same time period.
Harry realises his error in the 1.5 and says it “is not needed”.
Harry in about 2003 makes a new derivation of 1.5 based on errors in the estimation of sigma from sample standard deviations. For a special case of 30 points, p=.95 he multiplies Chi square factor by 3, subtracts 3 and gets “1.5”. The actual value ranges from 0 to 50+. He calls this a “correction”, not a shift.
Harry’s partner Reigle Stewart adds a new calculation he calls a “dynamic mean off-set.”: 3 / sqrt( n ) where 3 is the value for control limits and n is the subgroup size. For n=4 he gets “1.5”. Reigle says “This means that the classic Xbar chart can only detect a 1.5 sigma shift (or larger) in the process mean when subgroup size is 4″Reigle is quite incorrect. Such data is readily available from ARL (Average Run Length) plots.
Furthermore six sigma tables are based on the assumption of normal distributions.  No data will ever exactly follow a normal distribution to the extreme six sigma tail. 
In summary, the 1.5 does not exist, despite the many attempts to prop it up. Calculations involving 1.5 are hence meaningless. That is, 3.4 dpmo is meaningless. Six sigma tables are meaningless. Sigma levels are meaningless.
DO NOT USE SIX SIGMA TABLES.

This is supposed to be a professional forum. The professor has told us why he thinks the 1.5 shift is rubbish.
Hacl and Darth should tell us why they think the 1.5 is not rubbish, rather than making foolish remarks.

A more important question is do you believe each defect is independent?

Well why don’t you answer his question?  Can you?  I’ve yet to see anything of substance from you. 

That is pretty stupid but as good an answer as any of Harry and Reigle’s !!!!

RE: “Dr. Harry has put severe constraints around his derivation of the 1.46 shift and has never assumed it to be a constant as it is often used.”

What are the constraints ?
How are the constraint levels calculated ?
What is the derivation or proof of these constraint levels ?
Do you have a link to the proof of the constraint levels ?

If the 1.5 doesn’t exist, how can it have constraints ?I’d love to see how Darth , hacl and the SS cronies answer BB.Crap + crap = more crap

“Praveen is not a real smart guy.”
Understatement of the week

Darth,
Could you please share with us how this book answers these basic questions.

Darth claims:  ‘All is revealed in his online book entitled “Mysteries of Six Sigma”.  “
Where are you Darth ?  Is Stan correct ?

Sorry, don’t intend to quote all the passages.  If you have a burning desire, order the book.  It will answer where he got the 1.46 rounded to 1.5, what caveats he puts around it, his assumptions and his feelings about it’s use as a universal truth.  Of course, you can argue with the math and assumptions but he is clear about not using it as a constant.  That seems to have come a long way from building tables with the shift built in.

Of course Stan is correct. What a silly question.

Yes, it seems that every joker with 6 months experience and a dumb notion that books make big money is writing a Six Sigma book.
I’ll bet those Six Sigma mysteries have a wide target audiance.
I have a rare copy of mysteries I would be willing to part with for $100.00 – should I put it on ebay?

My software lists Sigma Level & Sigma Capability.  Although I know this stuff well, I’m not sure what these mean.  Please help.
As far as professionalism goes, we work a lot of hours, and do not need the mental stress of Truman Show behavior.
Love, qa guy