The R-Bar chart is another control chart which is used to monitor control of a "range". The R-Bar control chart tracks the difference between the maximum and minimum values in a subgroup.
Readings on the R-Bar chart are created by gathering measurements from a sample which is usually small in size. Once sample data has been gathered, the highest value is identified and the lowest value is identified. The difference between these two values is then calculated and recorded. The R-Bar chart will then reveal a "range".