Velocity spread versus absorption redshift of the 87 systems analysed. Symbols are equivalent to those in Fig. ⁵. Strong Mg ii absorption profiles (black circles) show a homogeneous distribution with redshift, but very strong Mg ii absorption profiles (red squares) show a significant difference in their median value of velocity spread between low (z_abs < 1.2) and high (z_abs > 1.2) redshift.