\begin{table}%T8
\caption{Optically type~2~AGN fraction in few representative X-ray surveys.}\label{t8}
%\centerline
{
\begin{tabular}{lrrrr}
\hline
\hline
Sample                     & Energy range   & Flux range               & Type~2 fraction      & Ref./Note    \\
(1)                        & (2)            & (3)                      & (4)                  & (5)          \\
\hline
ROSAT Bright Survey          & 0.5--2.0      & $> \sim 2\times 10^{-12}$ &  $1 \pm 0.5$\%       & (a)      \\
XMM BSS                      & 0.5--4.5       & ${>} 7\times 10^{-14}$     &  $13 \pm 4$\%        & (b)      \\
XMM HBSS                     & 4.5--7.5       & ${>} 7\times 10^{-14}$     &  $34 \pm 9$\%        & (c)      \\
HELLAS2XMM + Other Surveys   & 2--10          & $ 10^{-15} - 10^{-14}$   &  $57 \pm 12$\%       & (d)      \\
HELLAS2XMM + Other Surveys   & 2--10          & $ 10^{-14} - 10^{-13}$   &  $37 \pm 6$\%        & (e)      \\
\hline 
\end{tabular}}
\par
\medskip
Columns are as follows:
(1) Sample;
(2) Energy Selection Band; 
(3) Investigated flux range;
(4) Optically type 2~AGN fraction over the total~AGN density in the specific survey;
(5) References and Note as follows:
    (a) From Schwope et~al. (\cite{Schwope00});
    (b) From this paper using the BSS sources with Right Ascension below $\rm 5^h$ or 
        above $\rm 17^h$. For this BSS subsample the spectroscopic identification rate is 
	around $88\%$;
    (c) From this paper using the HBSS sample and taking into account the small 
        number of sources still unidentified as discussed in Sect.~3.6;
    (d) Fraction of optically obscured~AGN from Fiore
        et~al. (\cite{Fiore03}). The spectroscopic ID rate of the sample in this flux range is 
	${\sim} 50\%$;
    (e) Fraction of optically obscured~AGN from Fiore
        et~al. (\cite{Fiore03}). The spectroscopic ID rate of the sample in this flux range is 
	${\sim} 81\%$.
\end{table}