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Goodhart’s Law is an adage often stated as, “When a measure becomes a target, it ceases to be a good measure”. It is named after British economist Charles Goodhart, who is credited with expressing the core idea of the adage in a 1975 article on monetary policy in the United Kingdom: Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes.
It was used to criticize the British Thatcher government for trying to conduct monetary policy on the basis of targets for broad and narrow money, but the law reflects a much more general phenomenon.