Editing Allan variance (section)

==Interpretation of value==
Allan variance is defined as one half of the time average of the squares of the differences between successive readings of the [[frequency deviation]] sampled over the sampling period. The Allan variance depends on the time period used between samples, therefore, it is a function of the sample period, commonly denoted as ''τ'', likewise the distribution being measured, and is displayed as a graph rather than a single number. A low Allan variance is a characteristic of a clock with good stability over the measured period.

Allan deviation is widely used for plots (conventionally in [[Log–log plot|log–log]] format) and presentation of numbers. It is preferred, as it gives the relative amplitude stability, allowing ease of comparison with other sources of errors.

An Allan deviation of 1.3{{e|−9}} at observation time 1 s (i.e. ''τ'' = 1 s) should be interpreted as there being an instability in frequency between two observations 1&nbsp;second apart with a relative [[root mean square]] (RMS) value of 1.3{{e|−9}}. For a 10&nbsp;MHz clock, this would be equivalent to 13&nbsp;mHz RMS movement. If the phase stability of an oscillator is needed, then the [[time deviation]] variants should be consulted and used.

One may convert the Allan variance and other time-domain variances into frequency-domain measures of time (phase) and frequency stability.<ref>http://www.allanstime.com/Publications/DWA/Conversion_from_Allan_variance_to_Spectral_Densities.pdf. {{Webarchive|url=https://web.archive.org/web/20120206212113/http://www.allanstime.com/Publications/DWA/Conversion_from_Allan_variance_to_Spectral_Densities.pdf |date=6 February 2012 }}</ref>