Can I Use Cpk to Calculate the Sigma Level Here?

Six Sigma – iSixSigma Forums General Forums Tools & Templates Can I Use Cpk to Calculate the Sigma Level Here?

This topic contains 3 replies, has 2 voices, and was last updated by  Strayer 1 year ago.

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    Nabi Darwazeh

    Dear iSixSigma
    I have a process where the USL is 100 units with no LSL. We collected one sample per day, 12 days per month. We repeated the process over many months. All the values were between zero and few units. I calculated the Cpk using the (USL – mean)/3sigma. The Cpk in this case was very large because the mean was much less than the USL (Cpk = 30). Is it logical to use this Cpk value to calculate the Sigma level, if so (3×30 = 90????). If not what is the alternative way to calculate the sigma level?
    Thank you in advance


    Chris Seider

    It means you don’t have to focus on improving this process for quality control issues. However, are there cost benefits to raising the mean much closer to the USL? Less processing, cheaper processing, fewer ….

    Just 2 cents.


    Nabil Darwazeh

    In this process, we are looking at purified water microbial counts. The maximum allowed level is 100 counts. The lowest the count, the better the water quality. Therefore, we do not want to increase the number of the microbial counts to have near the USL. My question was, under such condition, is the calculated Sigma level realistic?



    When you have a single-sided specification it makes little sense to calculate variation from the mean. Search on this site for “cpk with one sided tolerance” or look it up in a good statistics reference book.

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