SPC and Rational Subgrouping

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Thanks so much for reading our publication. We hope you find it informative and useful. Happy charting and may the data always support your position.


Dr. Bill McNeese
BPI Consulting, LLC

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Comments (3)

  • AnonymousFebruary 27, 2013 Reply

    It occurred to me while reading this newsletter that Plan B does have value, in conjunction with Plan A.  Let's say that you know from Plan A analysis that the performances of the four machines are acceptably close to each other, but their outputs have been "uniformly" unacceptable now and then.  Assuming that the inputs to all four machines comes from the same input stream, I'd be inclined to suspect a change in the input as the culprit.  Maybe I'm getting batches of raw material from different suppliers and one of the suppliers sends me bad batches, or maybe I have only one supplier but they sometimes use an alternate shipper who doesn't adhere to a transport requirement, et cetera…  Would not a Plan B analysis, then, better help me determine the dates when the "uniform" machine performance was outside acceptable limits?  With those dates in hand, I'd have a shot at identifying the culprit.

    • billFebruary 27, 2013 Reply

      It is possible, but plan A would also show that since each subgroup would move up or down if the input stream is the same.  Unless I misunderstand what you are saying.  The two links at the bottom to the 2005 newsletters show an example of where two machines (A and B) operate at one average and the other two machines operate at a different average.  If all four where the same and the input stream changed, I would expect all the subgroups to move in a similar fashion.

  • AnonymousFebruary 27, 2013 Reply

    Mr. McNeese,I learn’t something new about control charts. Enjoyed your article.Please continue the good work …Sincerely,Neville Divecha, BSME, MBA, PMP

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