The standard deviation σ of measurements of a quantity with respect to the mean is
With a normal or Gaussian distribution of data, 68.3% of the data fall within a standard deviation about the mean. The square of the standard deviation is the variance. See statistical measures.
For two degrees of freedom, measurements with respect to the means , is
For a Rayleigh distribution of data, 40.5% fall within a circle of radius (called one sigma).