ADAPTIVE-CONTROL LIMITS FOR BIVARIATE PROCESS MONITORING

Variables control charts implement process state testing by separately assessing the measures of central tendency and spread. This work, bas ed on the ideas of bivariate process control, analyzes whether the sta ndard charting method could be considered as optimal and presents an a lternative mode of process evaluation-biparameter description in terms of the joint distribution of sample averages a nd standard deviations . Thus it becomes possible to benefit from the advantages of this appr oach and at the same time to overcome its main shortcoming, namely lac k of diagnostic ability. The proposed single (X) over bar chart with a daptive control limits provides a reliable tool for recognizing proces s state and indicating the parameter responsible for an out-of-control state. Additionally, the proposed method does not require a constant sample size during data collection.