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Goodhart's law: why a number stops telling the truth once it becomes a target

Goodhart's law in one story. In 1902, the city of Hanoi paid people for every rat tail. So people cut off the tails and let the rats go. Some even bred rats. Numbers at work break the same way when they become a target. Learn why, with proxies, gaming, Campbell's law and perverse incentives, what to do instead, and what happens when an AI tries to get a high score.