The standard method of characterizing the frequency stability of oscillators in the time domain, both short and long term. The traditional variance describes the deviation of a set of observations from the mean, but is not defined for noise processes more divergent than white noise. The Allan variance is convergent for all noise processes associated with precision oscillators. It is fast and accurate in estimating the noise process, easy to compute and has a straightforward relationship to the power law spectral density types. The Allan Deviation is a square root of Allan variance.

Link to Allan Variance