If you go to the ROC plot, and select the Helper Tool "?" and click in the window, it will bring up the help window for the chart. There you will find that they reference
DeLong, E. R., DeLong, D. M., and Clarke-Pearson, D. L. (1988). “Comparing the Areas under Two or More Correlated Receiver Operating Characteristic Curves: A Nonparametric Approach.” Biometrics 44:837–845.
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