Adjustment rules based on quality control charts

SUMMARY When lack of control is detected on a control chart, the usual recommendation is to ‘search for an assignable cause’, or ‘take remedial action’, etc., without specific advice on how to proceed. One important situation is a continuously adjustable machine whose mean output is being monitored by a Shewhart -chart. When lack of control is indicated by a signal from this or some other type of control chart, a natural action is to make an adjustment to re-centre the process mean. In this paper we propose three simple methods for determining the amount of adjustment and report on analytical investigations of their properties Somewhat different from other literature in this area is our desire to avoid overadjustment of the process mean, viewing this as a more serious error than underadjustment.

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