@Alcaniz] for a discussion about this point).

This work is organized as follows. In section \[sec2\] we briefly describe the quasar sample investigated, while in section \[sec3\] we explain the methodology of our analyses. In section \[sec4\] we show our results and discussions. The conclusions and final remarks are presented in section \[sec5\]. We leave complementary analyses and robustness tests to the Appendix section.

The Quasars data set {#sec2}
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The data used in this work is part of the twelfth public Data Release Quasar catalogue (DR12Q) from phase III of the Sloan Digital Sky Survey (SDSS-III) [@Eisenstein2011]  [^1]. The complete sample of DR12Q contains $297,301$ quasars from the Baryon Oscillation Spectroscopic Survey (BOSS) [@Dawson2013], among which $184,101$ have $z \ge 2.15$ (actually, more than 90% of them are new discoveries), covering a total area of $9,376 \, \mbox{deg}^{2}$ in the sky. The full sample has been spectroscopically confirmed based in a visual inspection of the spectra of each quasar. The SDSS-III/BOSS limiting magnitudes for quasar target selection are $r \le 21.85$ or $g \le 22$ [@Paris]. Perhaps, the main challenge faced in the quasar BOSS survey was to obtain a high number-density sample, satisfying the proposed minimum threshold 