Column density distribution functions from synthetic (left) and real data (centre) at ⟨z⟩ = 2.5. Horizontal bars represent the bin over which f(N_HI,χ) is calculated and vertical error bars represent Poissonian uncertainty. The difference between output and input mock distributions is shown at the bottom of panel a). The double power-law and Γ-function fits to the DR7 distribution (N09, ⟨z⟩ = 2.9) are shown as red dashed lines. f(N_HI,χ)(z = 0) are taken from Braun (2012, purple) and Zwaan et al. (2005, green). Right: the contribution of DLAs in a given N(H   i) range to the total mass census of neutral gas. DR9 values are corrected for systematics.