A note on two process adjustment models

The idea of a process 'adjustment' has different connotations depending on whether one is talking from a statistical process control (SPC) or from an engineering process control (EPC) point of view. In SPC an adjustment implies the correction of some problem that has caused abnormal variation. The corrective action usually restores the process mean to a desired or 'in-control' level. In EPC an adjustment means the compensation of some observed deviation by manipulating a control variable. This paper analyses two models proposed for process adjustment useful for understanding these and other important differences. One model is based on assumptions common in EPC applications, the other is based on assumptions common in SPC applications. Adjustment policies for each model are presented and contrasted.