Centroid velocity at 1/2 flux intensity (c(1/2)) of C IVλ1549 vs. c(1/2) of [O III]λλ4959,5007. Blue and red open squares show the HE Pop. A and Pop. B sources studied by M09 and S17. Colour scheme is the same as in Fig. 4. The green line indicates the linear regression between c(1/2) of C IVλ1549 and [O III]λλ4959,5007 obtained through the orthogonal least square method. The error bars refer to 1σ level of confidence for our sample.