A measure of the variability in a process or sample. It is calculated as the difference between the minimum (lowest) value and the maximum (highest) value in the data set.
When expressed as the difference of the upper and lower specification limits (USL - LSL) for a given process characteristic, it is called the Tolerance range of the process.
X-Bar and R Chart
NIST Statistics Handbook: Measures of Scale - http://www.itl.nist.gov/div898/handbook/eda/section3/eda356.htm#VARIANCE