\begin{table}%t5
\caption{Mean metallicity and extinction per unit metal column density for the DLA-QSOs with \ion{Si}{ii}~measurements.}\label{tab5}
\small%\centerline
 {
\begin{tabular}{ccccccccc}
\hline \hline
Color & & & $(r-z)$   & & & & $(g-z)$     & \\
\hline
%&&&&&&&& \\[-8pt]
 Absorption 
 &   $n$   & ${\langle {\rm [M/H]} \rangle}^b$ & ${\langle {\rm [M/H]} \rangle}_w^c$ & $\langle s_{\rm V}^{\rm Fe} \rangle^d$  
 &   $n$   & ${\langle {\rm [M/H]} \rangle}^b$ & ${\langle {\rm [M/H]} \rangle}_w^c$ & $\langle s_{\rm V}^{\rm Fe} \rangle^d$ \\
  sample$^a$          &           &  &     & [10$^{-17}$ mag cm$^2$]          
   &           &   &   & [10$^{-17}$ mag cm$^2$] \\
\hline
1  & 180 & $-1.19 \pm 0.03$ & $-1.07$ & 14 $\pm$ 8 & 171 & $-1.18 \pm 0.03$ & $-1.07$ & 7.5 $\pm$ 4.3\\ 
2  & 179 & $-1.18 \pm 0.03$ & $-1.07$ & 12 $\pm$ 8 & 170 & $-1.18 \pm 0.03$ & $-1.06$ & 5.8 $\pm$ 4.0\\
3  & 178 & $-1.18 \pm 0.03$ & $-1.07$ & 11 $\pm$ 8 & 169 & $-1.17 \pm 0.03$ & $-1.06$ & 5.4 $\pm$ 4.0\\  
\hline 
\end{tabular}}
\medskip
$^a$ Absorption samples are defined in Table~\ref{tab3}. Only cases with \ion{Si}{ii}~line detected
at more than 1$\sigma$~level are considered in this table. \\  
$^b$ Mean metallicity $\langle {\rm [M/H]} \rangle \equiv \log ~ \langle 10^{\rm [M/H]}\rangle$. 
The quoted error is the standard error of the mean.\\  
$^c$ Mean \ion{H}{i} column-density weighted metallicity of Eq.~(\ref{HIWeightedDef}). \\  
$^d$ Mean extinction per unit dust column density of iron (Sect.~5.5).
The quoted error is the standard error of the mean.   
\end{table}