C IV column density, log N_CIV, versus the associated column density of neutral hydrogen, log N_HI. The vertical dotted lines mark the values of log N_HI = 13.0, 13.5, 14.0, and 14.8. Blue data points represent the detected C IV systems identified by D16; white triangles indicate the upper limits on log N_CIV, which we used to derive the mock measurements (see Sect. 4.3). The red triangles represent the upper limits in the range between log N_HI = 14.0 and log N_HI = 14.8, not included in our tests. The dashed green line indicates the relation log N_CIV = log N_HI − 2.5 which is used, in the range 13 < log N_HI < 14, to create the mock systems for one of our tests, see Sect. 4.3.