\begin{table}%t6
\caption{Parameters of the linear fits, weighted with errors, of the age and
metallicities (measured in different index-index diagrams indicated between
brackets) and the central velocity dispersion. The table shows the results for
the central values~($a_{\rm c}$ and~$b_{\rm c}$) and for the global values~($a_{\rm g}$ and~$b_{\rm g}$). The coefficients~$a$ and~$b$ represent the zero point and the slope of
the linear fit, respectively. The last column shows the $t$~parameter obtained
in a~$t$-test to check the hypothesis $b_{\rm c}=b_{\rm g}$. A~value of $t$~higher than~2.326 allows us to reject the hypothesis with a~significance level lower than~0.01.}\label{global.fits}
%\centering
\small
\begin{tabular}{@{}l rrrrr@{}}
\hline \hline
                        & \multicolumn{2}{c}{Central} & \multicolumn{2}{c}{Global} & \\
                        & \multicolumn{1}{c}{$a_{\rm c}$}   &\multicolumn{1}{c}{$b_{\rm c}$}&\multicolumn{1}{c}{$a_{\rm g}$}&
\multicolumn{1}{c}{$b_{\rm g}$}    & \multicolumn{1}{c}{$t$}     \\
 & & & & & \\[-8pt]
\hline
$[$M/H$]$ (CN$_2$--H$\beta$) & $0.335\pm 0.081$ &$  0.00037\pm 0.00034$ &$ -0.012\pm 0.091$ &$ 0.00080\pm 0.00038$ & 0.86\\
$[$M/H$]$ (Fe4383--H$\beta$) & $0.148\pm 0.064$ &$ -0.00050\pm 0.00030$ &$ -0.275\pm 0.226$ &$ 0.00041\pm 0.00087$ & 0.98\\
$[$M/H$]$ (Mgb--H$\beta$)    & $0.113\pm 0.075$ &$  0.00112\pm 0.00031$ &$ -0.427\pm 0.286$ &$ 0.00212\pm 0.00107$ & 0.88\\
log age                 & $9.534\pm 0.075$ &$  0.00177\pm 0.00033$ &$  9.950\pm 0.115$ &$ 0.00007\pm 0.00049$ & 2.88\\
\hline
\end{tabular}
\end{table}