English: A clock is most easily tested by comparing it with a far more accurate reference clock. During an interval of time τ, as measured by the reference clock, the clock under test advances by τy, where y is the (relative) clock frequency over that interval If we take three consecutive clock measurements as shown, we can get a value of (y-y')^2--a smaller value indicates a more stable and precise clock. If we repeat this procedure many times, the average value of (y-y')^2 is equal to the Allan variance (or Allan deviation squared) for observation time τ.