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Cpk Ppk results

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  • #44131

    Rodders
    Member

    Hya,
     
    I have seen on various sites Cpk limits, i.e.,
     
    Cpk< 1.0 = process not capable
    1.0 Cpk> 1.3 = capable process
     
    My question is, since Ppk is a more conservative approach, what would be the equivalent limits for Ppk?
     
    Thanks in advance
     
    Rodders

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    #140990

    Deep
    Participant

    Same, 1.33. If you want your process to deliver Six sigma performance, then you need your Ppk > 1.5 1.5 1.5Deep

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    #141002

    Belgus
    Participant

    Ppk takes into account long term process cabability with a 1.5 sigma shift over time.
    As for limits Cpk = 1.3 = a three sigma process ( that’s 95% of the data fits within 3 standard deviations from the mean. In terms of yeild then thats 93.3%
    An ideal six sigma process short term has a Cpk = 2
    So long term Ppk 1.45 = 4.5 sigma = 99.87% yield

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    #141007

    SPC
    Member

    According to AIAG, Minitab, and other reputable sources  Cpk is process capability based on est stdev from a control chart (process must be under control)   Ppk uses a calculated sample standard deviation.   Most guru’s (Wheeler) caution against the use of process capability indexes as the “voice of your process” for you six sigma folks until you have shown that the process is stable (SPC).  SPC, will allow you to detect a 1.5 sigma shift or greater to a confidence of 99 % within 10 subgroups if certain rules are used. Just like the warnings on the mutual funds “past performance does not guarantee future performance”  thus the importance of SPC. 

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    #141009

    Romel
    Member

    Cpk focuses on process capability of a sustained process/product while Ppk is a process capability that focus on a product/process under design. the goal for Ppk is 1.67

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    #141011

    Barry M. Nma
    Participant

    omg….that was terrible.
    Cpk indicates capability of the process based on short term variability and assuming an unvarying mu (it’s what the process would output if truly in control).
    Ppk indicates performance of the process over the long term, including varying/drifting mu throughout the run (it’s what actually piled up at the end of the process including all the variation truly present in the operation and all other warts and blemishes). 

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    #141021

    Darth
    Participant

    Same interpretation as what both of you have expressed.
    Cpk – process capability for short term
    Ppk – Capability for long term (includes the past – from design until long term)

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    #141022

    Darth
    Participant

    Fake Darth,
    Thank you for the response on my behalf since I am out of the country with limited access to the Internet.  But, please get your own posting name.

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