Evaluation
F1 score
F1 is the harmonic mean of precision and recall.
How it works
The harmonic mean is low when either component is low. F1 ignores true negatives, so its suitability depends on the task.
A concrete example
Illustrative: precision 0.80 and recall 0.90 give F1 ≈ 0.847.
Do not confuse it with Accuracy
F1 summarizes positive-class precision and recall; accuracy counts all correct decisions.