Line width vs. column density for the C iv absorption lines along 16 sightlines excluding Q0055−269 and J2233−606 at the two redshift bins. Error bars are fitting errors from the VPFIT profile fitting. In the upper panel, two heavy dashed lines delineate a 3σ detection limit for a spectrum with S/N = 120 and S/N = 90 per pixel. At 2.4 < z < 3.2, most spectra show S/N greater than 90. In the lower panel, the heavy dashed line shows a 3σ detection limit for S/N = 120. Broader and weaker absorption lines at the left of the detection limit are missed in lower S/N spectra. The vertical dotted line indicates the adopted low N_C iv bound of log  N_C iv = 12.2 above which the incompleteness does not affect the C iv detection significantly. The histogram shown with the base at log  N_C iv = 12.2 is the number of C iv lines as a function of b_C iv with the b_C iv binsize of 2 km s^-1. Thick ticks above the number distribution mark the median b_C iv for log  N_C iv ≥ 12.2. The total number of C iv lines is 194 and 171 at 2.4 < z < 3.2 and 1.9 < z < 2.4, respectively. Among them, 138 and 122 lines have log  N_C iv ≥ 12.2 at the same redshift range.