On enhanced interquartile range charting for process dispersion

A careful monitoring of process dispersion is necessary to get optimum output from any process. Control charts are very important process monitoring tools of Statistical Process Control toolkit. Interquartile range (IQR) is a famous dispersion measure that is used to monitor process dispersion in quality control literature. In this study, a set of auxiliary information-based Shewhart-type IQR charts are proposed for efficient monitoring of process dispersion under bivariate normal, t and gamma distributed processes. The control structures of the proposed charts are developed, and their performances are evaluated in terms of Average Time to Signal (ATS), Average Extra Quadratic Loss and Average Ratio to ATS. Comparisons are made among different charts to establish their superiorities for a quicker detection of process changes. An illustrative example is also provided to elaborate the procedural details of the proposed IQR charts using a real data set.