An individuals and moving range (X-MR) chart is a pair of control charts for processes with a subgroup size of one. Used to determine if a process is stable and predictable, it creates a picture of how the system changes over time. The individual (X) chart displays individual measurements. The moving range (MR) chart shows variability between one sample and the next. Individuals and moving range charts are also used to monitor the effects of process improvement theories.
The Moving range chart creates range values to plot by finding differences between consecutive samples.
Control charts are designed to help to understand and reduce variation in a process over time. More specifically, they are designed to minimize two common mistakes:
1. Adjusting a process when it would be better to leave it alone.
2. Not adjusting a process when an adjustment is likely required.
Control charts help to predict how a process will behave. If a reasonable number of data points all show as in-control on a control chart, you can make a useful prediction about your process:
Unless the process is changed in some way, it will continue to produce results centered on this mean and varying within these control limits.
If the control chart shows data points that are outside the limits or trends, or runs above or below the mean, this does not allow you to make a useful prediction about your process. This process is not stable and not predictable. Unless something is changed, we cannot be certain about the mean or the dispersion of data resulting from this process.
This predictive nature of control charts and their ability to minimize the mistakes described above are what makes them such valuable business tools.
When to use this chart:
1. When you want to see if your process is stable and predictable;
2. When you want to see how planned change affects a process;
3. When the time order of the data values is preserved;
4. When you have collected data in subgroups of one – one value at a time.
This chart is part of PI VIZpack™ by PQ Systems, a collection of eight process improvement charts for Power BI®. They provide a clear way to visualize and evaluate process behavior and performance. Get started with a free two-week trial today. After the trial, you may continue using the basic features or upgrade to the premium version. To learn more visit our website.