An I-MR chart is a statistical process control (SPC) tool that consists of two charts:
- The top chart (I) plots the individual measurements of a variable over time.
- The bottom chart (MR) plots a moving range of the data. It is used for processes with continuous data.
An I-MR chart is an industry standard for monitoring and controlling process outputs over time. In manufacturing, an I-MR chart is generally used for low-volume production and destructive or expensive testing. Many situations exist in transactional or business processes in which an I-MR chart can be used (for example, sales and inventory data). Generally, if you can obtain rational subgroups, you should use the Xbar-R or Xbar-S chart; otherwise, use the I-MR chart.
Answers the questions:
- How much common-cause variation does the process exhibit?
- Is the process stable over time?
- Did special causes exist during the timeframe of the plotted data?
- Does evidence suggest something has changed or the process is performing differently than expected?
- Does the mean of the process output change at different levels of a process input?
- Does the variation of the process output change at different levels of a process input?
- Do the dynamic patterns of the process output change at different levels of a process input?
|When to Use
||Assist in project selection by identifying outputs that exhibit high common-cause variation, frequent special causes, unstable variation, or other symptoms that point to the need for improvement.
|Start of project
||Verify process stability when performing a baseline capability analysis.
||Investigate effects of input variables on the process output over time.
||Verify process stability when performing confirmation runs after implementing improvements.
|End of project
||Verify process stability after implementing controls to obtain a final assessment of process capability.
|End of project
||Graphically compare the pre-project process dynamic behavior to the post-improvement dynamic behavior.
||Control inputs to the improved process after the project is complete.
||Monitor output of the improved process after the project is complete.
Your data must be values for continuous Y, but must not contain rational subgroups.
- Verify the measurement system for the Y data is adequate.
- Establish a data collection strategy to determine the best time interval for collecting data.
- Enter the collected data into a single column in the Minitab worksheet. Minitab can also directly import data from databases, text files, Excel, and so on.
- Optionally, Minitab can evaluate eight rules to determine if special causes are present.
- Optionally, you can identify meaningful process stages and input conditions in the chart by entering a categorical variable into an additional column. Stages have different center lines and control limits which help you make comparisons across stages. For example, you can examine changes in the process mean and variation before, during, and after the implementation of a new procedure on one I-MR chart.
For more information, go to Insert an analysis capture tool.