The symbol for the measurement of dispersion in a population is Sigma.
Related notes on dispersion:
We need six points (strata, units of measure) under the upper control limit of the range in order to have sufficient measurement discrimination. This is due to rounding. Too much rounding causes a loss of information about dispersion in a sample.
http://www.six-sigma-material.com/Measures-of-Dispersion.html (Measures of dispersion)