\begin{table}%t3
\caption{\label{tab.Xfactor}Average over the cloud sample of the $\ratiot$ factor for the different methods used to derive the molecular gas mass.}
\begin{tabular}{llrr}
\hline \hline \noalign{\smallskip}
{Method}& &\multicolumn{1}{r}{$\left\langle \ratiot\right\rangle$} & Range \\ 
& &\multicolumn{1}{r}{ ${\rm cm^{-2}/K\kms}$} &${\rm cm^{-2}/K\kms}$\\
\hline\noalign{\smallskip}
 {{8~{\rm \mum}}} &  by eye & $2.1\pm0.8\times10^{21}$ & $0.3-4.3\times10^{21}$\\
& {\tt CPROPS} & $3.5\pm1.4\times10^{21}$& $1.0-6.4\times10^{21}$ \\
%\hline
${\rm 160~\mu m \mbox{ with }}$ & by eye & $1.5\pm0.9\times10^{21}$ & $0.5-3.1\times10^{21}$ \\
${T_{\rm dust}\mbox{map}}$& {\tt CPROPS} & $1.3\pm0.9\times10^{21}$ & $0.4-2.7\times10^{21}$\\
%\hline
 {${M_{\rm vir}}^{{\rm }}$} & by eye & $1.6\pm0.8\times10^{21}$ & $0.2{-}2.8\times10^{21}$\\
& {\tt CPROPS} & $1.1\pm0.8\times10^{21}$ & $0.07-3.5\times10^{21}$\\
\hline
\end{tabular}
\tablefoot{Averages for the ``by eye'' and {\tt CPROPS} methods are respectively over~11 and 9~clouds. In the case of the virial masses the number of clouds in the sample are 12 and 11 respectively for the ``by eye'' and {\tt CPROPS} methods (see Table~\ref{tab.clouds}).}
\end{table}