\begin{table}%t6	
      \caption{Standard deviation of the spectral index difference.\label{tab:standev}}
$$
         \begin{array}{lcr}
         \hline\hline
{\rm Catalogue} & {\rm Percentage \ of\ sources\ with} & \sigma_{\rm SI} \\
{\rm name}  & {\rm identified\ spectrum} & \\
\hline
{\rm MITG} & 100\% & 0.26 \\
{\rm PMNS-S} & 100\% & 0.29 \\
{\rm AGN-QSO} & 87\% & 0.36 \\
{\rm PKS} & 100\% & 0.30 \\
{\rm FA87} & 91\% & 0.28 \\
{\rm 1Jy} & 100\% & 0.29 \\
{\rm USSR} & 99\% & 0.13 \\
{\rm PULSARS} & -  & -  \\
{\rm B3}  & 100\% & 0.10 \\
\hline
\end{array}
      $$
\end{table}