A Graphical Approach for Interpreting Out-of-Control Signals in Multivariate Control Charts

Dealing with several variables simultaneously is not a simple task for quality engineers because the interpretation of out-of-control signals involves the examination of the data from a multivariate perspective. In this paper, a graphical technique for interpreting out-of-control signals in multivariate control charts is proposed. This approach consists of a typical Hotelling’s chart with warning labels indicating:1) the univariate out-of-control points often masked by the T-squared statistic; 2) the plane of the biplot that should be investigated to identify the cause of the signal. The idea of using biplots as a complementary tool is to provide a broader view of the data without adding much complexity to the analysis. Finally, the benefits of this are shown through a case example of an automotive body assembly process.