# need help to calculate sixsigma

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• #26253

Vikas
Member

Sir;
I am trying to calculate six sigma by using following formula:
sixSigma=0.8406+SQRT(29.37-(2.221*(1/DPMO)))
WHERE DPMO=(611/1081)*1000000.
The answer comes is sixSigma=6.26 (which is wrong).
Please explain me what is wrong in my formula.
(0.8406,29.37,2.221 all I taking as a constant)
regards
vikas

0
#63793

Tajjammul Hussain
Member

Vikas,
Why are you complicating the calculation. You can either:
Use the DPMO to Sigma Conversion Table
OR
If you plan to do this on an Excel, then maintain a master table with sigma and DPMO values and use a Vlookup Function to give you the sigma value.
Hussain

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#63794

Vikas
Member

Dear Sir
I dont want to calculate sigma in Excel.
what is the formula to calculate sigma using dpmo?
regards
vikas

0
#63795

Tajjammul Hussain
Member

Ok…You just need to go one step back and look at yeild % and then calculate sigma using the following :
= Normsinv(yeild%) + 1.5 = Short Term Sigma

0
#63799

JC
Participant

Vikas,
Where did you get your fomula?  It looks like one of your colleagues regressed a portion of the Z table.   Assuming your data are long term, normally distributed, etc., I calculate a benchmark Sigma of 6.37 (let’s get real: 6.4). You calculated 6.3.  Not bad actually.  Using the Z table or Excell, I can get a more accurate estimate than you do, but do I have better information than you?  Not really.  You already have your yield.
If you have a few processes to benchmark, and you don’t have a Z table, try the Sigma Calculator on iSixSigma site.  If you need to crunch more (for a Monte Carlo analysis?) go high tech and use Excell.
Good luck.

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#63801

Slava
Member

You have forgotten to put a “ln” in your equation.
process Sigma = 0.8406 + SQRT(29.37 – 2.221 * (ln(DPMO)))
you can check it here – https://www.isixsigma.com/library/content/c020507a.asp

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#63808

CB
Participant

Calculate DPO first by using this formula:
DPO =        Total no. of defects / Total no. of units * no. of opportunities per unit
Then calculate DPMO =  DPO * 1000000.
Next step is to look at the corresponding table and find out the sigma. Short term and Long term.
Do let me know if you need the table.
All the best.

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#64816

Sudhi
Member

The formula being used is valid as long as the DPMO is <550000 (i.e <55% defects)  which would be the case in most situations.
In your case as mentioned it exceeds the limit (601/1081 = 56%)
So it is inappropriate to use this formula

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