Design Strategies for Individuals and Moving Range Control Charts

AbstractIndividuals 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 shortterm 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.KeywordsAutocorrelated ProcessAverage Run LengthBox-Cox TransformationMoving Range Control ChartsX Control Charts Additional informationNotes on contributorsSteven E. RigdonDr. Rigdon is Associate Professor and Chairman of the Department of Mathematics and Statistics. He is a Member of ASQC.Emma N. CruthisMs. Cruthis is Gas Planning Analyst in the Gas Department.Charles W. ChampDr. Champ is Associate Professor of Statistics in the Department of Mathematics and Computer Science. He is a Member of ASQC.