A two-sided SPRT control chart for process dispersion

Abstract

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.

Keywords:

• Average number of samples to signal
• average number of observations to signal
• parametric control chart
• statistical process control

Acknowledgments

The authors are grateful to the reviewer for his/her critical comments and suggestions that have greatly improved the contents of the paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

Shashibhushan B. Mahadik's work was supported in part by Shivaji University, Kolhapur through Research Strengthening Scheme (SU/C&U.D.Section/2/13/9). Dadasaheb G. Godase's work was supported by Department of Science and Technology through INSPIRE fellowship (IF160684).