Design Strategies for Individual and Moving Range Control Charts

Individuals and moving range charts are often used when production volume is too low to justify rational subgrouping. This article considers several aspects of the design strategy for these charts. We have found that the X chart alone is nearly as efficient as the combined X-MR chart for detecting changes in the process variability. We recommend, however, that the control limits on the individuals chart be based on a short-term estimate of the process variability, such as the moving range, rather than on a long term estimate, such as the sample standard deviation of the process. We also consider the effects of nonnormality and autocorrelation.