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DPPM Monitoring

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  • #34271

    eds
    Participant

    is it appropriate to use Control Charts in Monitoring Monthly DPPM? From where should i get the data to compute for the Control Limits? is 12 months data or 6 months data enough (of previous year) for Control Limit Computation? 

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    #94264

    DonB
    Participant

    I find monitoring DPPM with control charts useful, there are however some challenges.  You might reference Wheeler on this (http://www.spcpress.com I believe).  I origionally got the idea from one of his books.  I believe his guidelines on limits were that you can have “useful” limits with 6 data points – though not necesssarily accurate.
    It’s useful because:  you can really see when you have a true process change that has taken place-rather than just getting lucky for a day or a month.  You can also identify assignable causes and then work on these to make sure they don’t happen again.
    Challenges: 
    A) Keep in mind that if the DPPM is a combined DPPM from multiple processes (or multiple customers for example) then it will of course not be in-control and predictable until the sub-processes are in-control.  Of course getting the sub-processes in-control is the key.
    B) If you have 0 DPPM for periods, especially for multiple periods – calculation of the control limits will become very difficult and somewhat artificially low because the measurement system isn’t finding the variation.  Therefore processes with quite a few 0ppms are more difficult to work with than processes that constantly have defects.
    Good luck.

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    #94275

    marklamfu
    Participant

    The direct DMMP chart is not a variable control chart, generally, for process or product control , we perfer to use variable chart ( e.g. X_bar &R chart , IX_MR chart,etc.), DPPM chart is used to review the product performance achieved,
    If no chance to do variable control due to product nature (only attribute parameter) we also can use DPPM to control process or product qulity, at this situation, DPPM chart are similar with a np-chart or P-chart.
     

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