Dependence of C IV on different parameters. Left panel: relation between C IVc(1/2) and L/L_Edd. Right panel: C IVc(1/2) vs. L_bol (1450 Å). Grey and orange squares represent low-z Pop. A and Pop. B quasars respectively, while blue and red open squares indicate HE Pop. A and Pop. B sources from S17. Colour scheme is the same as in Fig. 4. Linear regressions are shown by the dashed lines. The errors refer only to our sample.