\begin{table}%t2
\caption{\label{table:02}Statistical properties of the samples.}
\small%\centering
\par
\begin{tabular}{l r c c c}
\hline\hline
Sample                & 
$N_{\rm obj}$         &
Median of 12+log(O/H) &
Mean of 12+log(O/H)   &
RMS  of 12+log(O/H)   \\
\hline 
Sample A             &   139 & 8.010 & 8.003 & 0.233 \\
Sample B             &   120 & 7.936 & 7.858 & 0.303 \\
Sample C             & 48~497 & 9.016 & 8.976 & 0.166 \\
\noalign{\smallskip}
Sample A+B           &   259 & 7.980 & 7.936 & 0.277 \\
\hline
\end{tabular}
\end{table}