\begin{table}%t3
\caption{Mean gradients of age, [M/H], and metallicity, derived with different
indices for LDEGs. $\sigma$: typical deviation; $N$: number of galaxies; 
$N_{\rm eff}$: effective number of points, $N_{\rm eff}=\left[\sum
(1/\sigma^2)\right]^2$/$\sum \left[(1/\sigma^2)^2\right]$; $t$: $t$-statistic to check the
hypothesis "mean~$\ne$ 0''; $\sigma_{\rm exp}$: typical deviation expected from
errors; $\alpha$: level of significance to reject the hypothesis
"$\sigma=\sigma_{\rm exp}$''. The last column 
contains the $t$-statistic used to test the hypothesis "mean grad[M/H]~= mean grad[M/H]'' 
(with different indices).}\label{mean-gradients}
%\centering
\small
\begin{tabular}{@{}lrrrrrrrr@{}}
\hline\hline
         & 
\multicolumn{1}{c}{Mean} & 
\multicolumn{1}{c}{$\sigma$} & 
$N$ & $N_{\rm eff}$ & $t$ & $\sigma_{\rm exp}$ & \multicolumn{1}{c}{$\alpha$} & $t$\\
\hline
grad~($\log$ age)              &$ 0.082\pm 0.015$ & 0.117  & 61  & 19.62&  3.1& 0.032& 1.63E$-13$ &   \\
grad [M/H]                    &$-0.206\pm 0.019$ & 0.151  & 61  & 17.25&  5.7& 0.075&  4.3E$-22$ &   \\
grad [M/H] (CN$_2$--H$\beta$)  &$-0.582\pm 0.032$ & 0.249  & 61  & 23.14& 11.2& 0.111&  2.6E$-31$  & 5.94\\
grad [M/H] (C4668--H$\beta$)   &$-0.459\pm 0.028$ & 0.219  & 61  & 21.78&  9.8& 0.078&  0.000     & 4.26\\
grad [M/H] (Fe4383--H$\beta$)  &$-0.197\pm 0.025$ & 0.197  & 61  & 15.31&  3.9& 0.109&  1.2E$-14$ & 0.14\\
grad [M/H] (Mgb--H$\beta$)     &$-0.407\pm 0.039$ & 0.302  & 61  & 14.53&  5.1& 0.138&  5.3E$-31$  & 2.30\\
grad [M/H] (Ca4227--H$\beta$)  &$-0.238\pm 0.034$ & 0.269  & 61  & 19.18&  3.4& 0.178&  2.9E$-7$  & 0.45\\
\hline
\end{tabular}
\end{table}