NB The number shown next to each hollow circle is the threshold value used to dichotomise PCR. Each set of circles are ordered in terms of threshold value, from a PCR of 0.13 for the circles farthest right, and a PCR of 0.50 for the circles farthest left. |

Figure 1: Pair of summary sensitivity and specificity results for each threshold value, plotted in ROC space, for three different models: (i) model (6) with a linear trend, (ii) model (6) extended to include a quadratic trend, and (iii) model (6) with a cubic trend (i.e. model (9)). |