# Six sigma

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• #42811

Aman
Participant

why we talk about six sigma why not 3 sigma, why not 7 sigma?

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#135333

Zero_Cool
Member

Aman,
If you could notice this is “ISixSigma Forum” not “ISevenSigma” nor “IThreeSigma”.  Well, anyway, we have touch this topic quite long time and your are quite a bit too late. Very very late. If you just excert an extra effort to seach this site, you will find how many discussion we have made in the past. Anyway, your late.
Zero_Cool

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#135336

Aman
Participant

Dear,
I think I have been misunderstood, what I meant to say was that …I wanna know the statisticl explaination of six sigma theory.
I know that would a very naive question but since I’m a biggner & I have to start somewhere. so please guys don’t mind.

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#135338

Maurizio
Participant

I see, the explaination is just that it is based on the variation from the the mean when you have a normal  (or chi, I never remember, should study more) distribution
I hope it helps at least to comprehend
Ciao

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#135365

Aman
Participant

Yes, I got the hint, but is there anyone who can elobrate it to us ( i mean me & other people like me).
But thanks alot for Maurizio

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#135390

Zero_Cool
Member

Aman,
A good question will deserve a good answer. Then:
As a layman’s shortcut explanation, Six Sigma is not totaly came from a statistical analysis though it uses statistic on the calculation of DPMO / PPM under the area of a normal distribution, the effect of 1.5 sigma shift is applied. The sigma shift is an empirical data from previous studies of those people pioneering SS methodology, from which they claim that it is from observed process long term variation of 1.4 to .16 sigma shift. Until now, with long discussion, this shift and the data of calculating the 1.4 to 1.6 sigma is not avialable. Thus, these shift are now merely an assumption that your process will shift as much as 1.6 sigma.
Fortunately, the methodology works fine, but the 1.5 sigma shift is ???? up to you to utilized.
Zero_Cool

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#135408

Maurizio
Participant

pardon my confusion, but when to talk about elaborating, what do you mean? do you want a formula that can calculate the sigma level and analyse it to see how it works. but I can tell you alsow that the sigma scale is not direct but it is logarithmic.
let me know what exactly you need and I will see to it that you have the answer, to the best of my abilities :)
Maurizio

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#135409

Vipin Kaushik
Member

Refer to my answer below the table. Table is put in to give you idea about the relationship between yield & the sigma level.

Six Sigma Conversion Table

Yield%
DPMO
Sigma

6.68%
933200
0

8.455%
915450
0.125

10.56%
894400
0.25

13.03%
869700
0.375

15.87%
841300
0.5

19.08%
809200
0.625

22.66%
773400
0.75

26.59.5%
734050
0.875

30.85%
691500
1

35.435%
645650
1.125

40.13%
598700
1.25

45.025%
549750
1.375

50%
500000
1.5

54.975%
450250
1.625

59.87%
401300
1.75

64.565%
354350
1.875

69.15%
308500
2

73.405%
265950
2.125

77.34%
226600
2.25

80.92%
190800
2.375

84.13%
158700
2.5

86.97%
130300
2.625

89.44%
105600
2.75

91.545%
84550
2.875

93.32%
66800
3

94.79%
52100
3.125

95.99%
40100
3.25

96.96%
30400
3.375

97.73%
22700
3.5

98.32%
16800
3.625

98.78%
12200
3.75

99.12%
8800
3.875

99.38%
6200
4

99.565%
4350
4.125

99.7%
3000
4.25

99.795%
2050
4.375

99.87%
1300
4.5

99.91%
900
4.625

99.94%
600
4.75

99.96%
400
4.875

99.977%
230
5

99.982%
180
5.125

99.987%
130
5.25

99.992%
80
5.375

99.997%
30
5.5

99.998%
23.35
5.625

99.998%
16.7
5.75

99.999%
10.05
5.875

99.99966%
3.4
6
Yield above this level would be very low even you put in lots of resource in it. I guess that’s why we talk about six sigma. Not more than six. Three we do not take as the yield would be very  low.
Hope I am there.
Regards,
Vipin

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#135497

Aman
Participant

Thanks alot Mazio,
I asked the reason why six sigma hy not some other value sigma? but I couldn’t get the right answer so far.
I need to know the theory behind this, also obviously as you go ( i mean you increase sigma) your decreasing the defects or non confomities. but why we stuck at SS level? why not some other value of sigma?

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#135510

Participant

Hi Aman
“Six Sigma” is the name of a quality improvement methodology.
It in no way implies that 6 Sigma is the target performance for every process, or indeed that 6 Sigma is good enough in every case.
Obviously some processes must be far higher than Six Sigma and for some, 3 or 4 sigma (or even 1 or 2 Sigma) may be good enough.
It is a balance between customer demands, business priorities, what  the competition are doing and the ROI of any improvement solution.
There are many discussions about why Six Sigma was chosen as the name, the most likely being (IMHO) the fact that it alliterates and therefore makes for good marketing (think Coca-Cola, Chupa Chips, Pet Palace and Freddy’s Fresh Fried Fish Factory). It is of course also a mark of high quality (3.4 defects per million) but achievable quality (7 Sigma is more like 2 defects in a billion).

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#135560

Maurizio Garzelli
Participant

indeed, I agree.
I was looking around in the internet and I found out for example, that a nice place with a good explaination for 6 sigma is wikipedia: http://en.wikipedia.org/wiki/Six_Sigma it has a nice article on 6 sigma.
the alliterative origin of it might be purely casual but it is quite nice ;)
Maurizio Garzelli

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#135561

kaiser_soze
Participant
#135562

TwoCents
Member

I agree with Adrian…I think the truth of the matter is that “Six Sigma” was selected because it’s catchy.
Whether you are 1.5 shift believer or not, I find it particularly interesting that the 1.5 z-shift conveniently closes the gap between the “catchy” Six Sigma (which is mathematically 2 parts/billion) and the more reasonably attainable 3.4 DPMO.
After all, would you spend a bunch of money on a program called “Four-point-Five Sigma”?

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#135570

Maurizio Garzelli
Participant

…not very 6 sigma oriented.

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#135595

Aman
Participant

Hi Maurizio GarzelliThanks alot for you & the others for their insight into that matter.
let me see the link that you have farwarded to us.

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