Ratios of the table cells to the marginal totals can then be used to
quantify:
Agreement: The proportion of warrens where the imagers agree on
presence or absence of warrens.
\begin{equation}
\frac{a+d}{a+b+c+d}\nonumber \\
\end{equation}False Nil: The proportion of warrens where imager “1”
detected entrances but imager “2” detected zero entrances.
\begin{equation}
\frac{b}{b+d}\nonumber \\
\end{equation}False Presence: The proportion of warrens where imager “1”
detected zero entrances but imager “2” detected at least 1 entrance.
\begin{equation}
\frac{c}{a+c}\nonumber \\
\end{equation}Note that the word “false” here is a value judgment given the
arbitrary decision of which imager to designate as “1”. However, use
of “visual” as the baseline to compare the performance of the thermal
imagers seems justifiable. For all analysis the significance level was
set at 0.05.