Best-fit slope and intrinsic dispersion from the regression analysis of the F_X − F_UV relation in narrow redshift intervals from z = 0.2 to z = 1.6. A linear fit of γ as a function of redshift in the form γ = m z + q gives m = −0.052 ± 0.018 and q = 0.673 ± 0.026 (dash-dotted blue line). Host-galaxy contamination at z < 0.7 mimics a spurious redshift evolution of γ. For a comparison, we overplot the resulting fit (magenta point) of the 13 local AGN in the redshift range 0.009 < z < 0.087, whose flux at 2500 Å is directly evaluated from their UV spectra.