= Solution
\b[The diagonal <means> no ranking discrimination against the reference labels, with ROC area 0.5.] At every operating point on that diagonal, the fraction of positives accepted equals the fraction of negatives accepted. A score independent of the true label has this expected behaviour.
This does not imply that exactly half of all individual predictions are correct: an imbalanced dataset can have high accuracy by always predicting the majority class. It also does not establish biological absence of signal; it describes what the evaluated score extracts relative to the chosen reference and dataset.
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