Economic design of VSI control charts

Abstract

Duncan considered the design parameters of Shewhart control charts in 1956 based on an economic view-point, and ensured that an economic design control chart actually lowers the cost compared with a Shewhart control chart. His research method has been widely used in subsequent studies on the subject and focused on a fixed-sampling interval (FSI). However, it has been discovered that variable-sampling-interval (VSI) control charts are substantially quicker than FSI control charts in detecting shifts in the process. In an earlier study, an economic design of a VSI control chart with a single assignable cause for a continuous flow has been proposed. This research extends a single assignable cause to multiple-assignable causes to construct an economic model of VSI control charts. A numerical example is provided to demonstrate the effectiveness of the proposed model. The result indicates that VSI is more effective than FSI control charts.

Keywords:

• Economic design
• Variable sampling interval
• x Control charts
• SPC

Acknowledgements

The authors wish to express appreciation for the support by National Sciences Council of the Republic of China, Grant No. NSC-93-2213-E-212-020. Financial assistance of NSERC of Canada to the second author for this collaborative research is greatly acknowledged as well. The valuable comments and suggestions of the reviewers are greatly appreciated.