What is the I-MR Chart?
Walter Shewart introduced the IMR chart; control charts are therefore also known as Shewart Charts. An I-MR chart (also called an XMR chart) is a combination of two charts (Individual Range and Moving Range), which are used to track process variability based on samples taken over a time period.
When continuous data are not collected into subgroups, an Individual Moving Range (IMR) chart can be used. Collect only one observation at a given time. A graphical I-MR chart shows the process variations over time. A graphical IMR chart can help identify when a process is out of control, and where to focus attention on the cause.
Definition of I-MR
The IMR chart is a combination of two charts: the Individuals (I-) chart and the Moving Range (MR-) chart.
I Chart: Individual chart displays individual data points and monitors the average and changes in the process as the data is collected regularly. This chart can help identify common causes and their assignable values in the process.
Moving Range Chart: The Individual chart monitors process mean while the Moving Range chart monitors process variation when data points are collected regularly. The moving range chart shows the absolute difference between each measurement from its previous measurement.
Use of an IMR Chart for:
- Check for any signs of causes that could lead to a process going out of control.
- Monitor the performance of the process before and after implementing process improvements.
- If the shop floor is slow and it’s difficult to get more samples.
- The cost of the measurement system is too high (like destructive tests).
- Most commonly used in batch processing.
What is the difference between an IMR chart and an X-R chart
The Individual chart shows the data at each point and tracks the shifts during the process. An X monitors the change over time.
The Moving Range chart tracks the absolute difference in each measurement compared to its previous measurement. The Range Chart tracks the change in the range of each subgroup over time.
When would you choose one over the other when comparing?
X – R chart should be used when there is a large amount of data available. An I-MR chart should be used if the data is limited. IMR chart should be used for subgroup sizes of one. The X-R chart should be considered for subgroup sizes between two and ten. X-S chart is appropriate when subgroups are eleven or larger.
