Testing an underlying assumption on a ROC curve based on rating data.

The receiver operating characteristic (ROC) curve is widely accepted as the measurement that best represents the accuracy of a diagnostic system that consists of a diagnostic imaging modality and a reader who is interpreting results from the modality. However, there is one implicit assumption in a ROC curve: that the cutoff points to declare a positive test for decision variables of a diseased patient and a nondiseased patient are the same. If this assumption is violated, then a ROC curve does not exist. The author develops a statistical procedure to test this assumption. To demonstrate, he applies the method to a ROC data set and shows that the assumption of the equal cutoff points could be rejected.