Predicted number of z ≥ 4 QSOs (both blazars and non-blazars) with the flux density at 1.4 GHz ≥1 mJy and mag_drop ≤ 21 in the sky area covered by the FIRST sample based on the observed number of blazars in the same area. The predictions are shown as a function of the parameter ‘p’, while the shaded area shows the Poissonian uncertainty on this number (dark blue = 1σ, light blue = 2σ). The horizontal dashed line indicates the observed number of sources, while the vertical lines report the best value of p (red solid line) together with the 2σ confidence interval (black short-dashed lines).