
\begin{center}
\begin{longtable}{cccc}
\caption{Frequencies in matched samples data. \label{SI:mcnemartab}} \\
 & & \multicolumn{2}{c}{Sample 1} \\
 & & present & absent \\
Sample 2 & present & $a$ & $b$ \\
         & absent  & $c$ & $d$ \\
\end{longtable}
\end{center}

