Past exam of the systems biology course of the University of Cambridge / 2024 / Paper 4 / Section I / 1.8 / Solution

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.