An R-chart is a type of control chart used to monitor the process variability (as the range) when measuring small subgroups (n ≤ 10) at regular intervals from a process.
Each point on the chart represents the value of a subgroup range.
The center line for each subgroup is the expected value of the range statistic. Note that the center line varies when the subgroup sizes are unequal.
For small subgroup sizes with k-sigma control limits the lower limit is zero in many situations due to the asymmetric distribution of the range statistic. In these cases, probability limits may be more appropriate.