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Capability statistics for between/within capability

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  • #47154

    Kiran Varri
    Participant

    How are “Capability statistics for between/within capability” different from “Overall capability statistics”?
    Please help
     
    Kiran Varri

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    #156875

    saeed
    Member

    short term vs long term

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    #156883

    Craig
    Participant

    “Within” :   Sigma is estimated from within group variation only. (Rbar / d2)
    “Between” :   Actually it is between and within! Sigma is estimated with all the data, not just the average range within groups. (Using the sum of squared deviations from the mean, divided by n-1, and then taking the square root.)
     

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    #156885

    Ruddy
    Participant

    Hi Kiran,
    Process drift is essential for computing your capability index.
    Between/Within Capability – that is the computed Cp or Cpk based on the samples you measured. Or also know as the “Short Term” Capability.
    Overall Capability is also known as “Long Term”, hence it is considering the process drift and variation in an overtime, normally at 1.5sigma.  
    Hope this will help.
    Regards.

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    #156895

    Craig
    Participant

    I’d have to disagree with the last post. Process drift isn’t essential for anything. I think the initial post is asking for the difference between PPK and CPK.
    It all boils down to how sigma is estimated. (free of assignable cause or otherwise)

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    #156897

    Messy Problem
    Participant

    I agree .. drift isn’t essential.
    The drift, shift, thing undermines the use of SPC and corrective action!

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