iSixSigma

CpK vs PpK

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  • #66889

    Dr Phil Whateley
    Participant

    Pp and Ppk are calculated in exactly the same way as Cp and Cpk. It is the determination of sigma that differs. For Cp and Cpk, sigma is determined as the process standard deviation for an in-control process (e.g. sigma = R-bar/d2 etc) THE PROCESS MUST BE IN CONTROL! For Pp or Ppk you can use the overall sample standard deviation (which includes any special cause variation)
    I would suggest you take a look at the QS9000 SPC manual for a detailed discussion of these indices, and when they are applicable.

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    #66904

    “Ken”
    Participant

    Patrick,

    I believe Phil covered the general calculation of these two indices quite well. In his note, Phil referenced what he called the “overall variation.” Some also call this the “total variation” of the process or system. You can calculate the total variation by determining the standard deviation of all the individual values collected in the capability study. The total variation does not require the data to be time ordered.

    The significance of Ppk vs. Cpk rests in the fact that Ppk compares the total process variation to the requirements, and does not require the data to be time ordered. Cpk compares the within subgroup, or inherent, variation to the requirements and does require the data to be time ordered.

    Please see some of the other discussions on this site for additional details.

    Ken

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    #27365

    Patrick
    Participant

    Could someone enlighten me on how to I calculate the PpK and its significant

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    #66919

    Praneet
    Participant

    Hi Patrick
    Dr Phil and Ken have given some very useful tips on the significance of Ppk & Cpk , I do not want to add to what has already been said ,but I would like you to go to the site below for getting a better understanding of what these indices are all about. The parent site is http://www.qualityamerica.com . I found this article very interesting. The link given below talks specifically of the significance of the Ppk index
    http://www.qualityamerica.com/knowledgecente/knowctrInterpreting_a_Process_Capabilit.htm

    Regards

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