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Goodhart's law: once a measure becomes a target, it stops measuring what it was meant to
Goodhart's law says that when a measure becomes a target, it ceases to be a good measure. The British economist Charles Goodhart expressed the core idea in a 1975 article on UK monetary policy, observing that any statistical regularity tends to collapse once pressure is placed on it for control purposes. The reason is that people and institutions respond to the rule rather than the thing it stands for. If a school is judged only on test scores, teachers teach to the test and scores rise while learning may not. The measure was a reliable signal only while nobody was trying to game it.
A measure is useful because it correlates with a goal it cannot directly capture, such as test scores standing in for learning. When people are rewarded for the measure, they find the cheapest ways to raise it, including ones that have nothing to do with the goal. The correlation breaks because the link between the number and the underlying aim was never causal.
The law explains why targets so often backfire in policy, business and public services. It is a warning that a number used to steer behaviour changes the behaviour, which undermines the number.
The same dynamic appears in many fields: hospitals that are rewarded for short waiting times can move patients around the queue, and companies chasing quarterly profit targets can cut long-term investment. Each case follows the same logic of a measure turning into the objective.
The law is often quoted in a shorter form, and it was popularised in later decades by researchers who applied it to fields far from economics, including sport, software and health.
From Wikipedia, the free encyclopedia · text CC BY-SA 4.0
Illustrative photo · Jamesfranklingresham, CC BY-SA 3.0, via Wikimedia Commons
