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.