Re-calculating thresholds

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    John Shepherd

    Hi – I have a question about re-calcutaing control limits.
    Imagine the following :
    (1) The process is stable and has been performing within control limits for a long period.
    (2) There is some concious change made to the process that alters the performance of that process.
    (3) I want to keep the revised process under statistical process control, so presumably I would need to re-set the performance of the new process as soon as I can.
    Question is … How can I set the thresholds of my revised process when I have little or no performance data upon which to work ?
    Or does this mean that my process goes out of SPC whilst I gather enough data to re-set the limits ? 
    (expanding the example .. if I collected process stats on a weekly basis, and my original process ran unchanged for 50 weeks, I would be fairly confident of the performance characteristics.  
    If in week 51 I change the process, how soon can I re-calculate the control limits ?)
    Regards    John


    Bob J

    Hi John,
    You will need to gather 25 subgroups after you revise the process before you can re-calculate the control limits.
    Best Regards,
    Bob J

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