Control chart for multivariate attribute processes

Many industrial processes are multivariate in nature since the quality of a product depends on more than one variable. Multivariate control procedures can be used to capture the relationship between the variables and to provi de more sensitive control than that provided by the application of univaria te control procedures on each variable. Much has been done on the multivari ate variable processes, such as embodied in control procedures based on Hot elling's T-2 statistic. However, little work has been done to deal with the control of multivariate attribute processes, which is very important in pr actical production processes. In this paper, we develop a Shewhart-type con trol chart to deal with multivariate attribute processes, which is called t he multivariate np chart (MNP chart). The control chart uses the weighted s um of the counts of nonconforming units with respect to all the quality cha racteristics as the plotted statistics. It enhances the efficiency of ident ifying the critical assignable cause when an out-of-control signal appears. It is also simple to interpret for out-of-control signals. The practical a pplication of the MNP chart is also discussed in this paper with an example presented to demonstrate the approach of the MNP chart and to compare with the univariate np control charts which are commonly used in industry.