Redshift distributions of the QSO catalogues. Left to right: eBOSS DR14 (orange), eBOSS DR16 (green), and DESI DR1 (blue). The DR14 distribution is broad (0.4 < z < 8) but sparse at the extremes; we use the well-populated range 0.4 < z < 2.8. The DESI DR1 distribution spans 0.8 < z < 3.5; we use 0.8 < z < 3.0 to avoid edge effects. All histograms use a bin width of Δz = 0.001. Note that the y-axis scales differ between panels. The primary overlap region for the eBOSS samples is 0.8 < z < 2.2, while DESI significantly overlaps in the range 0.8 < z < 2.3.