\begin{table}%T4
%\centering
\par
\caption{Nitrogen abundance determinations using different diagnostics from
  following papers: (1) \citet{vanzee98,pagel92}, (2) \citet{thurston96,pagel92}, (3)  \citet{shaw95}, (4) \citet{kewley02}. }
\label{tab:n_abun}
\begin{tabular}{ll}
\hline \hline
\smallskip
Diagnostic  &  $\log$(N/O) \\
\hline 
(1)  & $-0.98\pm0.23$ \\
(2)  & $-0.90\pm0.22$  \\
(3)  & $-0.98\pm0.13$ \\
(4)  & $-0.90\pm0.08$ \\
\hline
\end{tabular}
\end{table}