An I-MR chart plots individual observations (I chart) and moving ranges (MR chart) over time for variables data. Use this combination chart to monitor process center and 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.
A hospital administrator wants to determine whether the time to perform outpatient hernia surgery is stable. Because the data are not collected in subgroups, he uses an I-MR chart to monitor the mean and variation of the surgery times.
The points vary randomly around the center line and are within the control limits. No trends or patterns are present. The amount of time to perform hernia surgery and the variation in times are stable.