Fractional variability F_var as a function of frequency. X-axis error bars indicate the energy band of the instrument. Y-axis error bars denote uncertainty on the F_var value, and are smaller than the marker for most instruments. For FACT, Fermi-LAT, Swift/BAT, and MAXI, several markers are shown for different data-selection approaches: keeping all (orange), only > 2σ (black), or only positive (blue) data.