The main difficulties in the use of economic designs are the computations involved, the difficulty in specifying process parameters, and the fact that the sampling interval h rarely is a natural quantity of time. The economic design of nX-charts involves the determination of the optimum values of the three control parameters: the sample size n, the control limit coefficient k and the sampling interval h. This paper deals with the economic design of nX-charts for multiple assignable causes and develops an explicit equation for h in terms of n and k. With the explicit equation for h, the paper can give an optimization procedure, which is implementable in real time on a personal computer, for obtaining the optimal solutions of these parameters n,k and h for such charts. The results and the execution times of all numerical examples show that the present optimization procedure is accurate and efficient.
