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

Abstract

Recently, a MIX S 2 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 S 2 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 S 2 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 S 2 control chart, the VSI chart yields better performance in terms of less expected cost per unit of time.