The I-MR chart is an industry standard for monitoring and controlling process outputs over time. In manufacturing, the 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 the I-MR chart can be used (for example, sales and inventory data). Generally, if rational subgroups can be obtained, you should use the Xbar-R or Xbar-S chart; otherwise, use the I-MR chart.
|When to Use||Purpose|
|Pre-project||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.|
|Mid-project||Investigate effects of input variables on the process output over time.|
|Mid-project||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.|
|Post-project||Control inputs to the improved process after the project is complete.|
|Post-project||Monitor output of the improved process after the project is complete.|
Continuous Y, no rational subgroups
If you have discrete numeric data from which you can obtain every equally spaced value and you have measured at least 10 possible values, you can evaluate these data as if they are continuous.