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What if Cpk is 0.89

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This topic contains 7 replies, has 6 voices, and was last updated by  Ruddy 12 years, 4 months ago.

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  • #47145

    Mohsin Siddiqui
    Participant

    If I calculate CPk for a process and it is found to be 0.89, so what does it really shows?
    Can i calculate the rate of rejection from this value? or amount of rejection from this value??
    and how can i find the repeatibility of my process??
    pls help!!!

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    #156850

    Fake Gary Alert
    Participant

    Your  process  is  not  capable and  not  centered 

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    #156860

    CT
    Participant

    CpK of .89 means that your process is not centered.
    Calculate Cp = (USL – LSL)/6*Std.Dev
    To Calculate Non Conformint Rate: NCR=2P(Z<-2.67) Assuming Cp is .89 and same as your CpK value. To find P(Z<-2.67) you must have index table for Z
    In this case P(Z<-2.67)=.00379256 Thus, for a centered process with a Cp of .89 the nonconforming rate is 7585.12 ppm
    Once again this calculation assumes that your Cp and Cpk are the same.
    Hope this helps
    CT

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    #156861

    Jim Shelor
    Participant

    Mohsin,
     
    You cannot tell the reason for the low capability of your process by looking only at Cpk.  You need to examine both capability indices (Cp and Cpk) and both performance indices (Pp and Ppk) to determine the problem.
     
    Cp = What is my process capable of doing assuming the process is centered and all process variation is common cause variation?
     
    Cpk = What is my process capable of doing using the actual process mean and assuming all process variation is common cause variation?
     
    Pp = What is my process capable of doing assuming the process is centered and using actual total process variation?
     
    Ppk = What is my process actually doing using the actual process mean and the actual total process variation?
     
    To determine the reason for a low process capability or a low process performance, the following comparisons are used;
     

    If Cp >> Cpk – a mean centering problem exists.
     

    If Cp >> Pp – a total variation problem exists.
     

    If Cpk > Ppk – a total variation problem exists.
     

    If Cp >> Ppk – both a total variation and a centering may exist.
     
    If any of these indices are <1, your process is not capable of meeting your customer’s requirements.  Any of the indices being <1 will cause Ppk to be <1.
     
    I hope this helps.
     
    Jim Shelor

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    #156873

    Fake Gary Alert
    Participant

    Do  you  think  that  we  can  calculate Cp,Cpk,Pp and  Ppk for  the same process data,regardless the  special  status  of  each data distribution (sub-groups or  individual  data)??

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    #156877

    Jim Shelor
    Participant

    All 4 indices are calculated using the same set of data.  The data can be sub-grouped to calculate all four, or it can be individual to calculate all four.
    You cannot use some indices calculated from sub-grouped data and other indices calculated from individual data for any single analysis of all four indices.
    I am not sure what you are driving at with your question.
    Jim Shelor

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    #156880

    Moshin
    Participant

    Why do you think my process is not centered?

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    #156884

    Ruddy
    Participant

    hello moshin,
    if your Cpk is at 0.89, then definitely your process is not capable. what you could do is to check again your process based on the following :
    1. shift of your distribution – is it shift going to higher side of the specs or lower.
    2.  check for chronic factors involving your distribution.  factors that leads to defects.
    3.  repeatability is one of the major contributions for non capability of the process.  your product may be actually within specs but your equipments of process maybe erratic and sometimes gives misleading results. Use GRR.
    4.  go back to your 5m’s-1e and identify contributions leading to rejections.
     
    hope these would help
     
    regards.

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