Detecting a shift in variance using economically designed VSI control chart with combined attribute-variable inspection

Abstract

Recently, a MIX S² control chart using both attribute and variable inspections is proposed by Ho and Quinino (2016) to monitor the process variability. This paper considers the variable sampling interval (VSI) MIX S² control chart in which the sampling interval between two successive sampling points is varied based on the value of the preceding complementary sample variance to improve the performance of the original MIX S² control chart. Additionally, a unified economical design model for the process is established with the cost function derived. The performance of the VSI chart is compared to that of the original static chart. The numerical results show that comparing to original MIX S² control chart, the VSI chart yields better performance in terms of less expected cost per unit of time.

Keywords:

• Statistical process control
• Variable sampling interval
• Economic design
• Attribute and variable control charts

Acknowledgments

The authors greatly appreciate the comments of the editor and the referees, which were very helpful in improving the paper. This research is supported by the Nanhu Scholars Program for Young Scholars of XYNU and the Key Scientific Research Project of Colleges and Universities in Henan Province.