and R chart is a type of scheme, popularly known as control chart, used to monitor the mean and range of a normally distributed variables simultaneously, when samples are collected at regular intervals from a business or industrial process.

and R chart may suffer when the normality assumption is not valid.

The "chart" actually consists of a pair of charts: One to monitor the process standard deviation (as approximated by the sample moving range) and another to monitor the process mean, as is done with the

and R chart plots the mean value for the quality characteristic across all units in the sample,

, plus the range of the quality characteristic across all units in the sample as follows: The normal distribution is the basis for the charts and requires the following assumptions: The control limits for this chart type are:[2] The chart is advantageous in the following situations:[3] As with the

chart is only valid if the within-sample variability is constant.

chart is examined to determine if the sample mean is also in statistical control.

If on the other hand, the sample variability is not in statistical control, then the entire process is judged to be not in statistical control regardless of what the