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Control Chart

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  • #49029

    Singh
    Participant

    Dear friends,
                      Happy New Year. Will I be right in fixing the control limits after observing my data for certain duration because otherwise based on the value of quality characteristic these limits will keep on changing and perhaps the objective of tracking the data to identify the presence of special cause variation will not be fulfilled completely, though one may still see the nonrandom distribution of data? Your in put will help in getting me conceptualized it in a better way.
    Regards.
    Kanwal
     

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    #166801

    Westfield
    Member

    My advice would be that if you are certain that you have sufficient data points (minimum 12, preferably at least 20) you should lock that down as your Learning Data, including locking down the mean and the control limits. That will then give you a point of comparison for Test and then Live data, in order to show that there has been a process change, signalled by Special/Assignable causes according to one or more of the rules.
    Good luck
     

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    #166808

    GDS
    Participant

    Kanwal,
    I always try and get at a minimum 30 data points and lock that in as my baseline control limits and after that the control chart rules then apply. You can investigate any out of control points.
     
    One thing my supervisors and managers have problems with is when their mean shifts (Good or Bad) on a process they want to change the control limits, however they do not know why the mean shifted.I will not recaluculate control limits unless they can explain why the mean shifted.
    GDS

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    #167038

    Marky Mark
    Participant

    Hello,
    I would personally not fix the limits until you are satisfied that you have process stability. When you have this then you can move to SPC.

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