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Just a note

This shows the distribution of OD readings for both the known negatives and known positives. In this case the distributions are reasonably symmetrical. Unfortunately, as shown in the first graph, the 95th percentile cut-off point (shown in dashes) causes a substantial misclassification of known positives as negatives.

The second graph shows an alternate, much less biased, way of locating the cut-off point - equivalent to the 2 graph ROC plot, below.