\begin{table}%t2
\caption{\label{tab2}The results of the $\Sigma-D$ and $L-D$ fits of PN samples defined using the different methods of distance determination.}
%\centerline
{\small \begin{tabular} {llccccc}
\hline \hline \noalign{\smallskip}
No.& Sample$^{a}$& $\beta_{_{\Sigma - D}}^b$  &$r_{_{\Sigma- D}}^b$& $\alpha_{_{L-D}}^b$ & $r_{_{L - D}}^b$ & $N^b$\\
\noalign{\smallskip} \hline \noalign{\smallskip}
1 & Trigonometric Parallax & $1.79 \pm 0.22$ & $-0.90$ & $-0.21 \pm 0.22$ &\p00.23  & 18\\
2 & Trigonometric Parallax \dag & $2.02 \pm 0.18$ & $-0.95$ & \p00.02 $\pm$0.18 &$-0.02$  & 17\\
3 & Geometrical (trig. parall. + expan.) & $2.26 \pm 0.26$ & $-0.86$ & \p00.26$\pm$0.26&$-0.19$ & 30\\
4 & Geometrical (trig. parall. + expan.) \dag  & $2.44\pm0.22$ & $-0.91$ & \p00.44$\pm$0.22  &$-0.36$  & 29\\
5 & Gravitational& $1.42\pm0.37$ & $-0.66$ & $-0.58$ $\pm$0.37 &\p00.33 & 22\\
6 & Gravitational  \dag  & $2.53\pm0.31$ & $-0.89$ & \p00.53$ \pm 0.31$ &$-0.38$ & 20\\
7 & Spectroscopic  & $2.57 \pm 0.36$ & $-0.91$ & \p 00.57$ \pm 0.36$ &$-0.43$  & 13 \\
8 & Extinction  & $1.69\pm0.38$ & $-0.80$ & $-0.31 \pm0.38$ &\p00.25 & 13\\
9 & USNO-PN  & $2.38\pm0.56$ & $-0.90$ & \p00.38 $\pm$0.56 & $-0.32$ & \p06\\
 \hline
\end{tabular}}
\medskip
$^{a}$ For the first eight samples, PNe are extracted from the SSV and Ph02; for
USNO-PN -- see Table~\ref{tab3}; $^b$ the parameters of the fit~($\beta$ and $\alpha$, respectively) and the correlation coefficient $r$ are given for each sample. The number of PNe in the sample is given in the last column~($N$).
\end{table}