\begin{table}%t2  
\caption{\label{table_obs}Comparison of observational average $\left\langle L_{\rm X}/L_{\rm UV} \right\rangle_{\rm obs}$ ratios of the different sub-samples with model $\left\langle L_{\rm X}/L_{\rm UV} \right\rangle_{\rm model}$ ratios.}
%\centerline
 {\small \begin{tabular}{l c c c}        
\hline\hline \noalign{\smallskip}                
Sample  (number of objects) & $\left\langle \alpha_{\rm OX} \right\rangle$ &  $\left\langle L_{\rm X}/L_{\rm UV} \right\rangle_{\rm obs}$ &  $\left\langle L_{\rm X}/L_{\rm UV} \right\rangle_{\rm model}$\\    \hline  \noalign{\smallskip}
   Low-redshift Seyfert 1  (37) & $-1.34^{(1)}$ & 0.39  & 0.40 $\dot{M}_{0}/M_{8}$ \\  
   COMBO  (47) & $-1.36^{(2)}$ & 0.34 & 0.40 $\dot{M}_{0}/M_{8}$ \\    
   BQS  (45) & $-1.46^{(2)}$ & 0.19 & 0.17\\             
   SDSS main  (155) & $-1.51^{(1)}$ & 0.14 & -- \\      
   High-redshift luminous AGN  (36) & $-1.72^{(1)}$ & 0.04 & 0.01 $\dot{M}_{1}/M_{9}$  \\
   Most luminous QSO  (33) & $-1.80^{(3)}$ & 0.02 & 0.01 $\dot{M}_{1}/M_{9}$\\
\hline 
\end{tabular}}
\smallskip
 $^{(1)}$ Sample from \citet{S_et_2005};  $^{(2)}$~sample from \citet{S_et_2006};  $^{(3)}$~sample from \citet{J_et_2007}.
\end{table}