Airport Scanner Mental Model
Each bag either really has a prohibited item (
+) or it doesn’t (
–). You decide to
Flag (predict “knife”) or
Clear (predict “no knife”).
TP: Flagged + (correct catch)
TN: Cleared – (correct rejection)
FP: Flagged – (false alarm)
FN: Cleared + (dangerous miss)
Tip: Missing a real knife (FN) is worse than a false alarm (FP). Watch how metrics respond as the base rate changes.