“The noise” is a function of the averaging time (also called “measurement time” or “tau”), as is illustrated above.  For the same oscillator, the fluctuations in the frequency vs. time plot measured with a 0.1 second averaging time are larger than when measured with a 1 second averaging time.  Also shown are the corresponding Allan deviations.
	At short averaging times, the longer the averaging time, the lower the noise, up to the “flicker floor,” i.e., for certain noise processes (see the next four pages), the hills and valleys in the frequency vs. time data average out. Longer averaging does not help when the dominant noise process is flicker of frequency.  At the flicker floor, the Allan deviation is independent of averaging time.  At longer averaging times, the Allan deviation increases because the dominant noise process is random walk of frequency, for which the longer the averaging time, the larger the Allan deviation.