Measuring what a system catches and misses.
THE DEFINITIONS
Precision: of the items flagged, how many were correct
Recall: of the items that should have been flagged, how many were
WHY ACCURACY ALONE MISLEADS
If one per cent of transactions are fraudulent, a system predicting no fraud ever is ninety-nine per cent accurate and useless.
THE TRADE-OFF
Increasing one usually decreases the other.
A fraud system catching everything also flags many legitimate transactions.
WHICH MATTERS MORE
Depends on the cost of each kind of error.
Missing a fraudulent transaction versus blocking a legitimate customer.
Missing a disease versus an unnecessary test.
That is a business decision, not a technical one.
WHAT TO ASK A VENDOR
Not "how accurate is it" but "what does it miss, and what does it flag wrongly".
RELATED TERMS
Benchmark, false positive, model evaluation.
