Allan variance is a measure of frequency stability in clocks, oscillators, and amplifiers, expressed mathematically as and abbreviated as AVAR. It can be expressed like so:
Allan deviation
Like standard deviation, Allan variance has its deviated variant called Allan deviation (abbreviated as ADEV), derivable through the square root of AVAR: .
ADEV is typically plotted using a log-log linear graph of ADEV against observation time (). The plot suggests how the stability of a signal changes over time.