A DYNAMIC ECONOMIC-MODEL FOR AN (X)OVER-BAR-CONTROL CHART DESIGN

Parkhideh and Case (IIE Transactions, 21, 313-323 (1989)) developed an economic model for a dynamic (x) over bar-control chart. In developin g the model they considered six decision variables in their design met hodology. It therefore became very complicated to obtain the optimal c ombination that resulted in the minimum loss-cost. This note proposes an alternative and simplified design methodology that reduces the numb er of design variables from six to three. The optimal values are obtai ned by imposing the following constraints. The optimal sampling interv al h(i) (i = 1,2,...) is chosen such that the integrated hazard rate o ver each sampling interval is constant. The optimal sample size n(i) ( i = 1,2,...) is chosen such that the relative sample size per unit tim e during each sampling interval is constant. Analogously, the optimal control limit coefficient k(i) (i = 1,2,...) is chosen so that the pow er of the control charts remains constant over each sampling interval. The process failure mechanism is assumed to follow a Weibull shock mo del and the product quality characteristic is considered to be normal. Computational experience indicates that the proposed dynamic nonunifo rm control chart design is much simpler and provides a lower cost than that of Parkhideh and Case's dynamic model.