Rare events and the denominator of a flag

High sensitivity does not establish that most threshold flags are correct. This educational methods note uses a synthetic waveform archive to distinguish sensitivity, false-positive rate, precision, false-alarm ratio, and overall accuracy. Natural-frequency tables demonstrate how a rare target can yield more incorrect than correct flags, while a second population shows how precision changes with the base rate even when conditional rates are held fixed. A stricter invented threshold makes the retrieval and review-burden trade-off explicit. Primary reasoning and classifier-evaluation studies accompany forecast-verification definitions. The note also addresses event units, correlated windows, threshold tuning, and human review. It validates no detector, recommends no operational threshold, and explicitly excludes medical, navigation, emergency, and other safety applications.