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Control charts separate common-cause variation (inherent to the process) from special-cause variation (due to assignable factors). An X̄–R chart monitors process mean and dispersion: plot subgroup means on the X̄ chart and ranges on the R chart; investigate points outside control limits or non-random patterns.

Understanding the difference between "chance causes" (inherent process variation) and "assignable causes" (specific, rectifiable problems). X̄cap X bar Statistical Quality Control Book By M Mahajan Pdf Free