Quasar by quasar line number density evolution of C iv-enriched H i absorbers. The left panels are derived from the Δv_C iv = ±100 km s^-1 sample, while the right panels represent the Δv_C iv = ±10 km s^-1 one. The open circles in the upper panel represent C iv-enriched absorbers having log  N_H i = [12.75,14.0] and the filled circles in the lower panel represent log  N_H i = [14,17]. The vertical error bars mark 1σ Poisson errors, while the x-axis error bars show the redshift range covered by each sightline. Sightlines having no C iv-enriched H i absorbers for a given velocity range are plotted at log  ddn_C iv/ddz = 0. The solid lines represent linear regressions to the data, using the parameters summarised in Table 4. Sightlines with no C iv-enriched absorbers, log  dn_{H i + C iv}/dz is plotted to be 0 with a downward arrow.