Plots the moving range over time to monitor process variation for individual observations. Use the moving range chart to monitor process variation when it is difficult or impossible to group measurements into subgroups. This occurs when measurements are expensive, production volume is low, or products have a long cycle time. The moving range chart is also known as the MR chart.
Consider the moving range chart below. A hospital wants to track whether the variability in the amount of time to perform outpatient hernia surgery is stable over 2 weeks.
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The points vary randomly around the center line and are within the control limits. No trends or patterns are shown. The variability in the amount of time to perform hernia surgery is stable. The center line is the average moving range of all subgroups.
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When data are collected as individual observations, you cannot calculate the standard deviation for each subgroup. The moving range is an alternative way to calculate process variation by computing the ranges of two or more consecutive observations.
