Left: TPCF computed on the synthetic spectrum (with same S/N as the original one) in which we included the C IV systems associated with log N_HI < 14.0. Right: TPCF for the case in which we also added the 135 mock measurements derived from Eq. (4) in addition to the weak systems. The shaded coloured regions indicate the 1 σ and 3 σ uncertainty regions obtained from the distributions of TPCF values computed on the corresponding sets of mock spectra.