Goodhart's law

Goodhart's Law states 'When a measure becomes a target, it ceases to be a good measure.' It originated from Charles Goodhart's 1975 article on UK monetary policy, noting statistical relationships break down under control pressure. Related concepts include Campbell's Law, the Lucas critique, and rational expectations. The law applies widely: in education, teachers pushed students ahead for funding; in science, the h-index's correlation with awards decreased after widespread use; in healthcare, reducing length of stay led to premature discharges; in COVID-19 testing, a 100,000 test target was met but many tests weren't useful. The law warns that metrics used as targets become unreliable due to gaming and unintended consequences.