\begin{table}%t7
\caption{Mean stellar population gradients and their errors for HDEGs.
$\sigma$: standard deviation about the mean; $N$: number of galaxies averaged;
$N_{\rm eff}$: effective number of points; $t$: $t$-statistic to verify the
hypothesis "mean~= 0''; $\sigma_{\rm exp}$: standard deviation expected from the
errors;  $\alpha$: significance level to reject the hypothesis
"$\sigma$~= $\sigma_{\rm exp}$'' in a~$\chi^2$~test. Column~9 shows the
$t$-statistic to confirm whether the gradients obtained with individual
indicators are the same as the metallicity gradients obtained with a~combination of ten
different indicators. As a~reference, the 10th~column shows the mean
gradients obtained for the LDEGs from Sect.~\ref{sec.meangradients} (see
Table~\ref{mean-gradients}). The final column of the table shows 
the $t$-statistic, to verify if the mean gradients are the same for 
both samples of galaxies, LDEGs and HDEGs.
A~high value of~$t$ indicates significant differences.}\label{table.mean.coma}
%\centering
\small
\begin{tabular}{@{}l rrrrrrrrrr@{}}
\hline\hline
 & & & & & & & & & & \\[-8pt]
                             &\multicolumn{1}{c}{Mean}
                             &\multicolumn{1}{c}{$\sigma$} 
                             &\multicolumn{1}{c}{$N$} 
                             &\multicolumn{1}{c}{$N_{\rm eff}$} 
                             &\multicolumn{1}{c}{$t$} 
                             &\multicolumn{1}{c}{$\sigma_{\rm exp}$} 
                             &\multicolumn{1}{c}{$\alpha$} 
                             &\multicolumn{1}{c}{$t$} 
                             &\multicolumn{1}{c}{LDEGs}
                             &\multicolumn{1}{c@{}}{$t'$}\\
 & & & & & & & & & & \\[-8pt]
\hline
 grad log age                &$ 0.027\pm 0.067$ & 0.258  & 15 & 9.79 & 0.34 &
 0.138 &  0.0003 &      &  $0.082\pm 0.015 $& 0.8\\
 grad [M/H]                  &$-0.328\pm 0.064$ & 0.247  & 15 &11.85 & 5.59 &
 0.157 &  0.0017 &      & $-0.206\pm 0.019$& 1.8\\
 grad [M/H] (CN$_2$--H$\beta$)&$-0.643\pm 0.111$ & 0.432  & 15 &10.86 & 5.60 &
 0.236 &  0.0003 & 2.10 & $-0.582\pm 0.032$& 0.5 \\
 grad [M/H] (C4668--H$\beta$) &$-0.599\pm 0.092$ & 0.358  & 15 & 9.18 & 5.71 &
 0.185 & 4.2E$-06$ & 1.96 & $-0.459\pm 0.028$&1.4 \\
 grad [M/H] (Fe4383--H$\beta$)&$-0.168\pm 0.118$ & 0.456  & 15 & 8.95 & 1.47 &
 0.275 &  0.0014 & 0.95 & $-0.197\pm 0.025$&0.2\\
 grad [M/H] (Mgb--H$\beta$)   &$-0.537\pm 0.158$ & 0.611  & 15 &10.23 & 2.85 &
 0.337 & 4.2E$-05$ & 1.02 & $-0.407\pm 0.039$& 0.8\\
 grad [M/H] (Ca4227--H$\beta$)&$-0.168\pm 0.214$ & 0.831  & 15 &11.16 & 0.70 &
 0.434 & 1.7E$-06$ & 0.61 & $-0.238\pm 0.034$& 0.3\\           
\hline
\end{tabular}
\end{table}