An economic–statistical model is developed for variable parameters (VP) &Xmacr; charts in which all design parameters vary adaptively, that is, each of the design parameters (sample size, sampling interval and control-limit width) vary as a function of the most recent process information. The cost function due to controlling the process quality through a VP &Xmacr; chart is derived. During the optimization of the cost function, constraints are imposed on the expected times to signal when the process is in and out of control. In this way, required statistical properties can be assured. Through a numerical example, the proposed economic–statistical design approach for VP &Xmacr; charts is compared to the economic design for VP &Xmacr; charts and to the economic–statistical and economic designs for fixed parameters (FP) &Xmacr; charts in terms of the operating cost and the expected times to signal. From this example, it is possible to assess the benefits provided by the proposed model. Varying some input parameters, their effect on the optimal cost and on the optimal values of the design parameters was analysed.
