\begin{table}%tb.1
%\centering
\par
\caption{\label{mean.grad}$\alpha$ coefficient and corresponding error
obtained using the samples  of SB06 and S\'anchez-Bl\'azquez et~al. (2007).}
 { \begin{tabular}{lrr}
\hline\hline
Index       &$\alpha$~~~~~~~~~~~~~ &  Type\\  %& Mean   & Std  \\
\hline
$\sigma$    &$ 0.058\pm 0.047$ &   1   \\
D4000       &$ 0.027\pm 0.018$ &   1   \\
H$\delta_{\rm A}$ &$-0.721\pm 0.508$ &   2    \\
H$\delta_{\rm F}$ &$-0.206\pm 0.249$ &   2    \\
CN$_1$      &$ 0.058\pm 0.023$ &   2     \\
CN$_2$      &$ 0.059\pm 0.030$ &   2     \\
Ca4227      &$ 0.047\pm 0.075$ &   1     \\
G4300       &$ 0.027\pm 0.022$ &   1      \\ 
H$\gamma_{\rm A}$ &$-0.850\pm 0.422$ &   2     \\   
H$\gamma_{\rm F}$ &$-0.436\pm 0.321$ &   2     \\   
Fe4383      &$ 0.061\pm 0.033$ &   1     \\  
Ca4455      &$ 0.097\pm 0.075$ &   1     \\  
Fe4531      &$ 0.040\pm 0.021$ &   1    \\   
Fe4668      &$ 0.119\pm 0.049$ &   1    \\   
Fe5015      &$ 0.05 \pm 0.02$  &   1    \\   
H$\beta$    &$-0.05\pm  0.19$  &   1    \\   
Mgb         &$ 0.048\pm 0.051$ &   1    \\   
\hline
\end{tabular}}
\end{table}