Regression analysis of the L_X − L_UV relation for the final, entire selected SDSS-CSC2.0 (yellow) and Chandra COSMOS Legacy (blue) sample (Γ = 1.7−2.8, k δ_obs = 0.6), while in grey is shown the pre-selected parent sample (2332 SDSS-CSC2.0 (3430 observations) and 273 Chandra COSMOS Legacy sources), before the selection in the X-ray band is applied (the observed dispersion in this case is δ_obs = 0.32 dex). The use of spectroscopic data leads to a final dispersion of 0.24 dex, in agreement with the one found in previous works on samples of similar size using photometric data.