Fraction of the sky (top: |b|  < 10°, bottom: |b|  > 10°) covered by the BAT survey as a function of the 15–150 keV detection limiting flux for a detection threshold of 4.8 standard deviations. Different colours refer to different survey epochs. The insets show the limiting flux achieved for 50% of the sky as a function of time; the best fit is a power law consistent with t^-0.5.