The Moving Median Range (MMR) is a variation of the more common Moving Range (MR) chart. The MR chart is used to monitor the variability of a process by calculating the difference between consecutive data points in a time-ordered sequence. The MMR chart, on the other hand, uses the median of a group of consecutive data points instead of the difference between them.
The MMR chart is typically used when the process data is not normally distributed, or when the data contains outliers or non-random patterns that can affect the results of the MR chart. The MMR chart helps to detect changes in the variability of a process and to identify when a process is in control or out of control.
To create an MMR chart, the data is first divided into groups of a certain size, usually between 3 and 10 data points. The median value of each group is then calculated and plotted on a chart over time. The range between the highest and lowest median values is also calculated and plotted on the same chart as the Median Range Limits (MRLs), which help to identify when the process is out of control.
The MMR chart is useful in identifying changes in the process variability over time, and can help identify the causes of these changes. It can also be used in combination with other statistical process control charts, such as the X-bar and R charts or the X-bar and S charts, to provide a more comprehensive analysis of a process or system.