C iv column density distribution. Filled circles are our results at the redshift range used for the high-order fit C iv-enriched H i sample at 1.9 < z < 3.2. The CCD gap in the C iv region was accounted for. Red filled triangles and blue filled diamonds are from Pichon et al. (2003) at 1.5 < z < 2.3 and 2.3 < z < 3.1, respectively. Green open squares are taken from Songaila (2001) at 2.90 < z < 3.54. The vertical error bars indicate 1σ Poisson errors, while the x-axis error bars show the N_H i range covered by each data point. The black dotted line shows the linear regression to filled circles at log  N_C iv = [12.25,15.5]: log  dn_C iv/(dN_C ivdX) = (11.41 ± 1.61) + (−1.85 ± 0.13) × log  N_C iv. The solid line is the power law fit at log  N_C iv = [12.25,13.5]: log  dn_C iv/(dN_C ivdX) = (6.60 ± 1.23) + (−1.47 ± 0.10) × log  N_C iv. The turn-over at log  N_C iv ~ 12.5 shown in green data is simply due to the incompleteness for weak C iv. Similarly, the turn-over seen at log  N_C iv ~ 12.1 in our data is also due to the incompleteness.