FIGURE 2-2 A representative plot of the receiver operating characteristic (ROC) curve for a diagnostic test with accuracy index (A) of 0.8, showing three threshold or cutoff values: F, a “friendly” threshold; B, a “balanced” threshold, with equal probabilities of false positive and false negative errors; and S, a “suspicious” threshold.

NOTE: The diagonal line represents an accuracy index of 0.50 (chance). The point P represents an accuracy index of 1.00.

ROC, as indicated in Figure 2-2. This area, denoted A, is the accuracy index used in this book.7 Its possible range is from 0.5 at the “chance” diagonal to 1.0 for perfection. Figure 2-3 shows the ROCs for three values of A, 0.7, 0.8, and 0.9, and for the chance diagonal (0.50), under the assumption that the distributions of evidence follow a particular (Gaussian) symmetric form. Higher values of A indicate tests with greater accuracy. The curves for such tests are above and to the left of those for less accurate

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