The SPC literature does not consider what happens when an organization successfully uses its tools. It stabilizes unstable processes so that disruption from assignable causes becomes increasingly rare. While this happens, the false alarms from the common causes remain at the same frequency, and the ratio of true to false alarms drops to a level that destroys the credibility of the alarms.

This is a signal that further quality improvement can only be pursued with other tools, typically the conversion to one-piece flow to accelerate the detection of problems and, once human error becomes the dominant cause of defects, error-proofing. This article digs into the details of how this happens with control charts.