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The Polygraph and Lie Detection
from a value at the extreme right of the decision axis (no tests diagnosed as positive) to a value at the very left of that axis (all tests diagnosed as positive). If truth is known, these proportions can be used to estimate two probabilities: the conditional probability of a positive test result given the presence of the target condition (this probability—90 percent in the figure—is known as the sensitivity of the test) and the conditional probability of a positive result given the absence of the condition (which is the complement of the test’s specificity—and is 50 percent in the figure). The second panel shows that the proportions of false negative and true negative results, respectively, are complements of the first two and add no additional information. They do not, therefore, require separate representation in a measure of accuracy.
Receiver Operating Characteristic (ROC)
Figure 2-2 presents a representative function that shows the true positive rate (percent of deceivers correctly identified) and the false positive rate (percent of nondeceivers falsely implicated) for a given separation of the distributions of scores for all possible choices of threshold. The curve would be higher for diagnostic techniques that provide greater separations of the distributions (i.e., have higher accuracy) and lower for techniques that provide lesser separations (i.e., have lower accuracy). Such a curve is called a receiver operating characteristic (ROC). The ROC of random guessing lies on the diagonal line. For example, imagine a system of guessing that randomly picks a particular proportion of cases (say, 80 percent) to be positive: this system would be correct in 80 percent of the cases in which the condition is present (80 percent sensitivity or true-positive probability), but it would be wrong in 80 percent of the actually negative cases (80 percent false-positive probability or 20 percent specificity). Any other guessing system would appear as a different point on the diagonal line. The ROC of a perfect diagnostic technique is a point (P) at the upper left corner of the graph, where the true positive proportion is 1.0 and the false positive proportion is 0.
Measure of Accuracy
The position of the ROC on the graph reflects the accuracy of the diagnostic test, independent of any decision threshold(s) that may be used. It covers all possible thresholds, with one point on the curve reflecting the performance of the diagnostic test for each possible threshold, expressed in terms of the proportions of true and false positive and negative results for each threshold. A convenient overall quantitative index of accuracy is the proportion of the unit area of the graph that lies under the