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Questions re CpK and Ppk

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  • February 7, 2019 at 12:23 pm #236103

    Karim Mohsen
    Participant

    what is meant by Cpk is for short term, Ppk is for long term.? And
    Is Cpk tells you what the process is capable of doing in future,while Ppk tells you how the process has performed in the past?
    please help to understand this

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    February 7, 2019 at 5:38 pm #236114

    Patrick Detroit
    Participant

    Hi Karim, I don’t consider myself an expert, but I’ll try to answer the best I can. How the denominators are calculated for Cp/Cpk and Pp/Ppk will answer your question. The denominator for Cp/Cpk uses a pooled Stdev or Stdev(within) that is calculated by obtaining the square root of the sum of subroup stdev’s. The denominator for Pp/Ppk is the Stdev(overall), which is the stdev of the entire study. Cp/Cpk is focused on how the process behaves based on your samples and Pp/Ppk focuses on the entire process. Hope this helps.

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    February 7, 2019 at 5:56 pm #236115

    Chris Seider
    ★★★★★★★★★★
    Participant

    consider
    https://www.isixsigma.com/tools-templates/capability-indices-process-capability/process-capability-cp-cpk-and-process-performance-pp-ppk-what-difference/

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    February 7, 2019 at 10:28 pm #236129

    Patrick Detroit
    Participant

    Hi Chris,
    Maybe you can answer a question I was recently asked. Why is Cp/Cpk a 2.0 ratio? I explained the math, that the USL-LSL is a measure of +-6 sigma (12 sigmas) over the denominator that is only 6 sigma. My follow on question… is there a reason we use a 2.0 scale? If we used a denominator of 12 sigma the scale would be just as understandable (0.17; 0.33; 0.5; 0.67; 0.83; 1.0). I tried looking online and have come up empty handed. Thanks

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    February 8, 2019 at 5:47 am #236130

    Chris Seider
    ★★★★★★★★★★
    Participant

    Cp is meant to tell you the ABILITY to produce good product so it’s just a ratio of 99.7% of the “normal” distribution relative to the specs.

    Cpk is meant to give you an idea of how well your process is ACTUALLY doing relative to the closest spec to the mean–which is the highest portion of your defectives.

    There isn’t a ratio of 2.0 between Cpk and Cp because they are comparing different things in the numerator. Good question. Last point…it’s always important to graphically look at how your process is doing relative to the specs.

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