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Process Capability Non Normal – subgroup size

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  • #47523

    Tierradentro
    Participant

    Conducting an analysis of capability using the Box-Cox transformation.  We are studying 13 pieces each with 274 measurment.  What would the subgroup be in this case 13 or 274?

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    #158468

    annon
    Participant

    Dont understand…you are working with non-normal data and want to  convert it using box-cox for the purposes of conducting a capability analaysis?

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    #158511

    Tierradentro
    Participant

    Hi Annon,
    Yes we are working with non-normal data.  Which needs to be used for a capability study.  My main interest is which are the subgroup each piece produced or the measurment made on it?
     

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    #158532

    annon
    Participant

    Still a tad hazy, but if you are working with 13 units, each with 274 defect opportunities, and you want to determine process capability, I would look at using either a a DPU and then calculating a RTY using the inverse natural log, or go straight with a DPMO. 

    Either approach is user-friendly and robust to non-normality
    With only 13 units, I would have concerns about sample relevence.  But that is just my gut opinion.
    There are also several other approaches available to you, other than transformation:

    Use Individual Distribution Identification in MTB and run the corresponding cap study based on the distribution type
    Use a cap method robust to non-normality (see above)
    Etc
    Also, it might be helpful if you to take a look at how you are defining defect opportunities.  Personally, I define it as an opportunity to add value, value being defined as:

    A physical change to the unit
    That the customer is aware of and willing to pay for
    which is done right the first time
    This often reduces the DO to a more workable level with a greater relevance to the customer viewpoint.  Again, just my opinion.  You have many options.
    Good luck.
     

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