iSixSigma

Cpk vs Ppk

  • This topic has 6 replies, 7 voices, and was last updated 14 years ago by Ron.
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  • #50005

    Dan B
    Participant

    Can somebody explain, in plain language, the difference between Cpk and Ppk?
     

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    #171780

    Cao Rongshun
    Participant

    Cpk is for short term, Ppk is for long term.

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    #171783

    Taylor
    Participant

    CpK-Short term capability
    PpK-Long Term Capability

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    #171784

    annon
    Participant

    Cpk and Ppk are both estimates of process capability.  Capability is essentially the porportion between the variability allowed by the customer (spec limits) and the variability in your process (std dev), and since the spec limits are given to us by our customer, the only thing we can change in this context is the type of std dev used in the calculation.
    Cpk uses short term estimates, those that are said to be devoid of special cause.  Since our process is rarely devoid of special cause, it is often useful to use long term std dev, which is gathered over a longer period of time and includes both between and within variabililty estimates…it relates more to actual process performance.
    Cpk is something of an idealic state, while Ppk is more represenative of what your customer is experiencing, it is their reality.

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    #171785

    Kannan
    Participant

    i second annon. Just to add. Process capability analysis entails comparing the performance of a process against its specifications. We say that a process is capable if virtually all of the possible variable values fall within the specification limits. Cpk assumes that the data  is normally distributed. The calc for Cp= ((USL-LSL)/6s).
    We can modify this equation slightly to account for off-center processes to obtain the Cpk index = min( ((USL-x)/3s), ((x-LSL)/3s) )
     
    you can learn this graohically @  http://elsmar.com/Cp_vs_Cpk.html

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    #171848

    melvin
    Participant

    I believe that the difference is in the way sigma is calculated.  Cpk uses sigma estimate based on Rbar/d2 for control chart.    Ppk uses all the individual data to calculate sigma using the standard formula for sigma.  After that difference, the calculation of both is the same.  Hope that helps a little. 

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    #171849

    Ron
    Member

    Cpk is short term PPk long term
     
    one sample versus 25 samples

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