Economic Statistical Design for Three-level Control Charts with Variable Sample Size

The problem of Statistical Process Control with some discrete level classification scheme, have been recently discussed by some authors. This paper presents economic and economic-statistical designs for variable sample size (VSS) of the three-level control charts. The expected cost per unit of time is derived with the use of the cost model developed for the control chart proposed by Costa and Rahim. The genetic algorithm is employed to search for the optimal values of model parameters. Finally, the expected cost per unit of time of VSS and fixed ratio sampling (FRS) three-level control charts is compared.