- Precision = TP / (TP + FP) — 'of what I flagged as positive, how much was right?'
- Recall = TP / (TP + FN) — 'of all actual positives, how many did I catch?'
- F1 score — the harmonic mean of precision and recall, useful when you need one number
- Spam filters favour precision; disease screening favours recall
Precision and Recall
Precision measures how many predicted positives were correct; recall measures how many actual positives were found.