This standard covers methods of describing random instabilities of importance to frequency and time metrology. Quantities covered include frequency, amplitude, and phase instabilities; spectral densities of frequency, amplitude, and phase fluctuations; and time-domain measures of frequency fluctuations. In addition, recommendations are made for the reporting of measurements of frequency, amplitude and phase instabilities, especially as regards the recording of experimental parameters, experimental conditions, and calculation techniques. Basic concepts and definitions, time prediction, confidence limits when estimating the variance from a finite data set, and confidence limits for frequency domain measures are covered in the annexes. The annexes also cover translation between the frequency-domain and time-domain measures, examples on how to calculate the time-domain measures of frequency fluctuations, and an extensive bibliography of the relevant literature. Systematic instabilities, such as environmental effects and aging, are discussed in IEEE Std 1193-1994 .