Quality Quandaries: Six Sigma, Process Drift, Capability Indices, and Feedback Adjustment by George Box and Alberto Luceno. This article is for the more statistically inclined.
The Six Sigma specification makes an allowance of 1.5 standard deviations for process drift. Simple ways in which a major part of such drift can be removed are given. These employ feedback adjustment methods specifically designed for SPC applications.
Feedback adjustment can be dangerous: tampering. In fact, I would say attempting it is likely to be tampering, unless those doing so are careful and knowledgeable. It might be wise to read Box and Luceno’s book on the topic – Statistical Control: By Monitoring and Feedback Adjustment if you are tempted to try.