Economic statistical process control for over‐adjusted process mean

An economic adjustment model of a process whose quality can be affected by multiple special causes, resulting in changes of the process mean by incorrect adjustment of the process when it is operating according to its capability. A statistically constrained adjustment model is developed for the economic statistical design of X¯ control chart to control the process mean affected by multiple special causes. The objective is to determine the design parameters of the X¯ control chart, which minimize the total quality control cost. A Markov chain approach is used to derive the model. It is demonstrated that the expressions for the expected cycle time and the expected cycle cost with multiple special causes are easier to obtain by the proposed approach than by extending or adopting that in Collani et al. Application of the model is demonstrated through a numerical example.