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Also called two-sample deviation, or square-root of the "Allan variance," it is the standard method of describing the short term stability of oscillators in the time domain.  It is denoted by sy(t),
where
The fractional frequencies,           are measured over a time
interval, t; (yk+1 - yk) are the differences between pairs of
successive measurements of y, and, ideally, < > denotes a time
average of an infinite number of (yk+1 - yk)2.   A good estimate can be obtained by a limited number, m, of measurements (m³100).  sy(t) generally denotes                   i.e.,
Allan Deviation
   At long averaging times, especially when the averaging time is a substantial fraction of the record length, “TOTALDEV” statistics yield better results than the two-sample deviation (see the Howe reference below).
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D. W. Allan, "Time and Frequency (Time-Domain) Characterization, Estimation, and Prediction of Precision Clocks and Oscillators," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-34, No. 6, pp. 647-654,  November 1987.

“Characterization of Clocks and Oscillators,” edited by D.B. Sullivan, D.W. Allan, D.A. Howe, F.L. Walls, NIST Technical Note 1337, March 1990.

D. A. Howe, “Total Variance Explained,” Proc. 1999 IEEE Int’l Frequency Control Symp., Joint Meeting EFTF-IEEE IFCS, pp. 1093-1099.