A two-sided SPRT control chart for process dispersion

This paper proposes a two-sided sequential probability ratio test chart for monitoring dispersion of a normally distributed process. Accurate approximate expressions that relate the statistical performance measures of this chart to that of its corresponding one-sided charts are obtained through simulations. An approach to design the chart having the specified in-control performance and the optimal out-of-control performances for detecting the particular upper and lower shifts that are desired to detect as efficiently as possible is provided. It is found through numerical comparisons that the overall statistical performance of the proposed chart is superior to that of the competitor charts in the existing literature. The application of the chart is illustrated through real-data on the thickness of dielectric polyimide coating layer that is applied on a wafer used in an integrated circuit.