We develop a statistically constrained economic adjustment model of a process whose quality can be affected by double special causes resulting in changes of the mean and the variance of the distribution of output by incorrect adjustment of the process when it is operating according to its capability. The (X) over bar and S control charts are thus used to signal the special causes. A renewal theory approach is first extended to derive the economic adjustment model which is used to deter-mine the design parameters of the (X) over bar and S control charts that minimize the statistically constrained long term cost from the mistakes of process over-adjustment and under-adjustment. It is demonstrated that the expression for the economic adjustment model is easier to obtain by the proposed approach than by other approaches. Furthermore, the solution of this model is an improvement to the economic design, since it has the required statistical properties and consider the effect of over-adjustment. Finally, an example illustrates the construction and the application of the proposed control charts.