An MR chart plots the moving range over time to monitor process variation for individual observations. Use the MR 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.
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.
For example, a hospital administrator wants to track whether the variability in the amount of time to perform outpatient hernia surgery is stable.
The points vary randomly around the center line and are within the control limits. No trends or patterns are present. The variability in the amount of time to perform hernia surgery is stable.