Study On Construction And Appraisal Of Process Capability Index For Non-normal Processes

Statistical process control (SPC) is one of the main tools for controlling processes and widely used in the manufacturing industry. Traditional SPC theory is based on a fundamental assumption that the process data are normal. However, in many conditions the process data are non-normal which makes traditional SPC theory invalid. Therefore, the study on construction and appraisal of process capability index for non-normal processes is a hot issue to be solved.This paper firstly introduces the basic theory and knowledge of process capability index and some issues in the study of non-normal process capability and describes how to handle non-normal processes. Then it discusses some important transformation methods for non-normal process capability, which are Johnson transformation and Box-Cox transformation methods, and does some competitions with these two methods. Study on the best transformation parameter has been done by analyzing some cases. All of these methods can be used directly in practice. This paper also introduces some other transformation methods, two new process capability indexes for non-normal processes and non-transformation methods——corrected weighted variance method. All these methods can contribute to the practice and do well to the non-normal processes.The paper is based on the main line of process capability analysis methods for non-normal processes. Statistics principle, mathematics derivation, comparative analysis and case study are employed to carry out the thorough examination to the theory of process capability analysis and control methods for non-normal processes. All of these construct a systematic evaluation for capability index for non-normal processes.