Left: TPCF computed on the spectrum in which we included the mock measurements derived from the upper limits in addition to the C IV systems associated with log N_HI < 14.0. Right: TPCF for the case in which we set the column density value of the mock C IV measurements such that log N_CIV = log N_HI − 2.5 in the range 13 < log N_HI < 14. The shaded coloured regions indicate the 1 σ and 3 σ regions obtained from the distributions of the TPCF values computed on the corresponding sets of mock spectra.