Study On Process Capability Analysis And Control Methods For Non-normal Processes

Statistical process control (SPC) has been become an effective form of quality application to promote quality management development and made a significant contribution to continuous quality improvement. 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. Under the circumstances, the traditional SPC theory becomes invalid. Therefore, the problem of process capability analysis and control methods in non-normal processes has become an urgent issue to be solved.The basic theory and knowledge of the characteristics and test methods for non-normal process, process capability analysis and control charts are first introduced in this paper. For practical points of view, non-normal data stability in manufacturing processes is studied with the principle of Mann-Whitney U test. Skewness Correction (SC) control chart and Scaled Weighted Variance (SWV) control chart application are studied to apply, separately. An analysis is made to compare SC control chart, SWVcontrol chart, Shewhart control chart, WV control chart. The comparative study shows that SC control chart as well as SWVcontrol chart can control non-normal processes well than other control methods. The paper presents a Modified Weighted Variance process capability index which is more robust than process capability indices proposed by BAI & CHOI and process capability indices proposed by WU. In addition, the practical application of estimators of truncated samples from the normal distribution is derived and illustrated, and the paper imports the truncated samples which are easy to appear in manufacturing processes or supplier management into process capability evaluation for the first time. It is scientific to fund the estimators of untruncated samples by the study of paper.The paper is commenced by the nature of non-normal data and based on the main line of process capability analysis and control 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. The theory base and application framework of process capability analysis and control methods in non-normal processes were found from several sets including non-normal process stability, non-normal process control methods, non-normal process capability evaluation as well as process capability evaluation of truncated samples from the normal distribution in order to perfect statistical process control system.