Stein, Philip (2002, ASQ)
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[pages 64, 68]
[Abstract from articles]
Small or not, process variation that is consistent and random - what we usually call common cause variation - cannot be reduced by feedback. Feedback is appropriate, though, to address special cause variation - changes that have systematic, identifiable causes. That's where the control chart comes in. It is the tool, the statistical filter, that enables us to differentiate between special and common causes. We can then feed back corrective action to adjust for special causes without the increase in random variation that would come from trying to feed back inappropriate corrections from common causes.