The chart is necessary in the following situations:
Where automation allows inspection of each unit, so rational subgrouping has no benefit.
Where production is slow so that waiting for enough samples to make a rational subgroup unacceptably delays monitoring
For processes that produce homogeneous batches (e.g., chemical) where repeat measurements vary primarily because of measurement error
The "chart" actually consists of a pair of charts: one, the individuals chart, displays the individual measured values; the other, the moving range chart, displays the difference from one point to the next. As with other control charts, these two charts enable the user to monitor a process for shifts in the process that alter the mean or variance of the measured statistic.
As with other control charts, the individuals and moving range charts consist of points plotted with the control limits, or natural process limits. These limits are the voice of the process, and define what the process will deliver without fundamental changes to the process. Points outside of these control limits are signals indicating that the... Read More