PERFORMANCE COMPARISONS OF R, CUSUM AND EWMA CONTROL CHARTS FOR MONITORING THE PROCESS VARIABILITY TO APPROACH SIX-SIGMA QUALITY

摘要

This work is based on the performance comparison of Range(R), CuSum and EWMA control charts for monitoring the process variability. When monitoring the process it is important to quickly detect increase and decrease in variability. If the variance of the process increases, it is possible that an increased number of units will be defective. Therefore, in this paper we consider only increase in the process variability. These three types of control charts are used to detect the changes in process parameters. Therefore, the object of this research work is to find out which of these three charts provides better performance (more efficient) in order to detect the changes in the increase of process standard deviation for Six-Sigma quality control. The Range(R), CuSum and EWMA control charts were used to determine the average run length(ARL) to detect out of control condition. The ARL found by these three charts and was compared to each other. It was observed that the performance of the CuSum and EWMA control charts for monitoring the process variability were appreciably better than the performance of R-chart when small shifts of interest. The CuSum and EWMA control charts are more efficient to detect the small shifts in the process dispersion at the order of shift level below than 1.5. While, if the larger shifts are considered (shifts in the process dispersion more than1.5), the R-chart was the most effective.