99.9% accuracy can mean zero discoveries: a professor explains the class imbalance trap

bravo_abad · x · 2026-09-17

A Spanish scholar (author of Inteligencia artificial para estudiantes de ciencias, Ediciones Pirámide, 2026) explains class imbalance with a classic example: with 10,000 samples and only 10 genuinely positive rare scientific events, a trivial classifier that always predicts 'negative' scores 99.9% accuracy while finding exactly zero of the events you care about. The takeaway: accuracy alone badly misleads on imbalanced data, especially in scientific discovery settings.

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