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Revise Control Limits present Sigma level

Six Sigma – iSixSigma Forums Old Forums Software/IT Revise Control Limits present Sigma level

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  • #26124

    AJ
    Participant

    Hi,We have a process where we need to calibrate a group of people to record customer call information based on set guidelines/parameters. As there are certain judgemental pieces of information, we have variation among different persons. Initially, we started taking the variation limits as +/-10.00%. Now, after we have some months of data, we checked the Cpk which was >1. Hence the limits were revised to +/-3 sigma (around +/-7%) with Cpk still >1. Can someone help me in clearing some doubts? 1) Can we find out the current sigma level? Is it 3 sigma? 2)How and when shall we revise the limits again?

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    #63477

    Bob J
    Participant

    Hi Aj,
    I’ll see if I can help…
    If I understand correctly, you have currently established +/- 3 standard deviations around the mean as your guideline.  Since your Cpk is based on the number of standard deviations between your mean and your nearest guideline, I am not surprised that it is approximately 1.  This is essentially the same technique used in control charting to determine whether a process is consistent and repeatable….
    Usually, the guidelines are established by the overall objective for the project or the needs of the customer.  As such, guidelines are usually established as a hard target and not as a number of standard deviations from the mean.  In this situation, the only time you change the standard is when the customer expectations change or a new improvement project has been established with a new goal. 
    Hopefully this helps with your second question…
    As for the first question, the answer is yes….  Assuming a stable process (which it looks like you have), the sigma level will be 3 times the Cpk value.
    Best Regards,
    Bob J

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