with the luminosity. As an example, in Fig.\[fig:IT\_hasinger\] the simulated X-ray Baldwin effect obtained using the model of @Ikeda:2009nx and the $\theta_{\mathrm{OA}}-L_{\mathrm{X}}$ relationship of @Hasinger:2008ve for $\log N_{\rm\,H}^{\rm\,T}=23.1$ is shown, together with the fit to the X-ray Baldwin effect obtained by the recent works of @Bianchi:2007vn and @Shu:2012fk. The spread in $EW$ for a given luminosity is due to the range of values of $\theta_{\,\mathrm{i}}$ we considered: larger values of $EW$ usually correspond to lower values of $\theta_{\,\mathrm{i}}$.

Most of the studies of the X-ray Baldwin effect performed in the last years have used a relationship of the type $$\label{eq:fit_XBE}
\log EW=A+B \log L_{\mathrm{X,44}},$$ to fit the $EW-L_{\mathrm{X}}$ trend, where $L_{\mathrm{X,44}}$ is the luminosity in units of $10^{44}\rm\,erg\,s^{-1}$. In order to compare the simulated the $EW-L_{\mathrm{X}}$ trend with that observed in unabsorbed populations of AGN, we fitted, for each value of $N_{\rm\,H}^{\rm\,T}$, the simulated data with Eq.\[eq:fit\_XBE\] using the weighted least-square method, with weights of $w=\sin \theta_{\,\mathrm{i}}$. This allows to account for the non-uniform probability of randomly observing an AGN within a certain solid angle from the polar axis.

The X-ray Baldwin effect {#sect:XBEslope}
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Most of the studies on the X-ray Baldwin effect have found, using Eq.\[eq:fit\_XBE\], a slope 