An Interactive Look at Process Capability

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Thanks so much for reading our publication. We hope you find it informative and useful. Happy charting and may the data always support your position.


Dr. Bill McNeese
BPI Consulting, LLC

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Comments (18)

  • pardeepJune 22, 2017 Reply

    This article is extremely helpful. Thanks a lot for writing this article. It answered almost all doubts I had on process capability. Remaining doubts I am writing here: how you calculate ppm out of spec for diff. Sigma levels. In above artical table no 2  for sigma level 1.5  there is out of spec lsl ppm is 3 and for usl ppm is 66807.  I want to know how you calculate this???

    • billJune 23, 2017 Reply

      Thank you.   In Excel you can calculate the ppm above the USL as:
      1000000* (1 – NormSDist((USL- Avg)) / Sigma))
      Similar for LSL.

  • RamDecember 18, 2019 Reply

    Hi, can i have step by step method how the cpk requirement set as 1.33 

    • billDecember 19, 2019 Reply

      I am not sure what you mean.  You can set the Cpk requirement to anything.  Common one is 1.33, which sets the specification 4 sigma from the average.  

  • PreethamFebruary 8, 2020 Reply

    Hi,Can u explain difference between sigma and sigma level

    • billFebruary 10, 2020 Reply

      Sigma level is a measure of process capability: the higher the process sigma, the more capable the process is.  Sigma is an estimate of the standard deviation of the process.

  • PreethamFebruary 8, 2020 Reply

    Hi,this excel formula u have given is working only for sigma value= 1, for less than 1 its showing ppm more than lakhs.

    • billFebruary 10, 2020 Reply

      Not sure what you are saying.  Lakhs?

  • WillMarch 28, 2020 Reply

    I work in retail distribution.  We're interested in using SPC for measuring whether our warehouse processes are in control or not.  Any advice for using these concepts for a physical movement of goods vs tolerance for a manufactured item?  Would appreciate any guidance.

  • Wen CHENNovember 18, 2020 Reply

    Hello,6 sigma level with 1.5sigma should be 4.5 sigma level, right? why it becomes 3 sigma level? can you pzl explain, thxBRWen

    • billNovember 18, 2020 Reply

      I am not sure what you are asking.  Please give me more information.

  • SamAugust 27, 2021 Reply

    Hi…Had a doubt on what is the difference between six sigma and process capability index

    • billAugust 27, 2021 Reply

      A six sigma process (3.4 defects per million) has a Cpk value of 2.0.

  • LakiNovember 25, 2021 Reply

    Can you please provide password, to learn the equiations used, will be very helpful. Thanks. 

    • billNovember 27, 2021 Reply

      There is no password. You can see the equations by unhiding the sheets.

  • AnonymousNovember 30, 2021 Reply

    please share calculation – how sigma value (e.g.1.33) is derived.

    • billNovember 30, 2021 Reply

      Do you mean Cpk?  The equations are in the article.  Sigmal level is 3*Cpk

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