# control chart for six sigma

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Viewing 7 posts - 1 through 7 (of 7 total)
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• #38399

S.Lakshminarasimhan
Member

control chart for 3 sigma is widley known. Can any one give information about a shewhart x bar and R chart for six sigma limts

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#114845

Kenny
Participant

How much understanding do you have on the theory behind control chart applications and why 3 sigma limits are used?

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#114847

S.Lakshminarasimhan
Member

As you could see from my posting I am talking only about Shewhart’s chart.  I know about ‘no correlation’,’intutive 3 sigma limitswithout shift’,’normality’,’phase 1′ and phase II’, ‘each point is a test of hypothesis’,’rational sub group’,’assignable causes’

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#114863

FTSBB
Participant

Suggest you pick up a copy of Understanding Statistical Process Control by Wheeler & Chambers & read chapter #4.  He has a good discussion regarding why 3-sig limits are used.  Also, he warns not to simply “adjust the control charts to 6-sig limits – the limits are NOT solely based on probability theory.”
Guess I was most curious as to why you were looking at 6-sig control charts.  Tough to argue with 80+ years of control chart use @ 3-sig…

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#114868

Ken Feldman
Participant

Good news, Minitab will let you put your control limits anywhere you want, 6 sigma or 20 sigma, doesn’t matter, just put’em in and she will print them out.  Remember Shewhart was trying to balance his type 1 and type 2 errors and concluded empirically that 3 s.d. was the most economical balance.  You can do what you suggest but why??

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#114875

S.Lakshminarasimhan
Member

why six sigma control chart, because in 3 sigma about 68000 defetives and only assignable cause controlled, but in six sigma about 3.4 defetives and all causes including common causes to be addressed

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#114877

Ken Feldman
Participant

Apples and oranges.  Get hold of Shewhart’s book or any good  book on control charts and read a little more.  Defectives and 3.4 have nothing to do with the Xbar/R chart you were asking about.

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Viewing 7 posts - 1 through 7 (of 7 total)

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