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.

本文提出了一种用于监测正态分布过程离散度的双侧序贯概率比检验图。将该图表的统计性能指标与其对应的单边图表的统计性能指标相关联的准确近似表达式是通过模拟获得的。提供了一种设计图表的方法，该图表具有指定的控制性能和最佳的失控性能，用于检测希望尽可能有效地检测的特定上下偏移。通过数值比较发现，所提出的图表的整体统计性能优于现有文献中的竞争对手图表。