\begin{table}%t3
%\label{tab:ratio}
\caption{Mean flux and SFR ratios for the different samples. Extremely discrepant points (if any) have been discarded before performing the calculation.}
\label{tab:fluxratiosamples}
%\centering
\par
 {\begin{tabular}{c c c c}
\hline\hline
Sample&$\left<{F_{\rm IR8.0}/(F_{\rm IR4.5}-\alpha F_{\rm IR3.6}})\right>$&$\left<\left[8.0\right]/\left[{\rm NUV}\right]\right>$&$\left<\left[{\rm H}\alpha\right]/\left[{\rm NUV}\right]\right>$\\\hline
NGC~5291&$21\pm17$&$0.5\pm0.4$&$0.5\pm0.2$\\
Rosenberg&$17\pm12$&--&--\\
SINGS&$25\pm16$&$3.3\pm7.0$&--\\
Engelbracht&$24\pm23$&--&--\\
M~51&--&$4.2\pm5.5$&$1.1\pm1.3$\\
M~81&$50\pm35$&$1.2\pm0.9$&$2.8\pm2.2$\\
Arp~82&$45\pm24$&$1.8\pm1.8$&$1.0\pm0.7$\\
\hline
\end{tabular}}
\end{table}