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.