Goodhart’s Law: when a measure becomes a target, it ceases to be a good measure.

Goodhart's law illustration showing a manager frustrated by 1000's of tiny nails when measuring on number of nails made, and pulling their hair out when presented with giant nails when measuring on weight

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Goodhart's law states that when a measure becomes a target, it ceases to be a good measure. In other words, if you pick a measure to assess performance, people find a way to game it.

To illustrate, I like the (probably apocryphal) story of a nail factory that sets "Number of nails produced" as its measure of productivity. The workers figure out they can easily make tons of tiny nails to hit the target.

Yet, when the frustrated managers switch the assessment to "weight of nails made", the workers again outfox them by making a few giant heavy nails.

And there's the story of measuring fitness by steps from a pedometer only to find the pedometer attached to the dog.

Some strategies for helping this are to try and find better, harder-to-game measures, assess with multiple measures, or allow a little discretion. More detail in this nice little article .

I also liked an idea I read in Measure What Matters of pairing a quantity measure with a quality measure, for example, assessing both the number of nails and customer satisfaction of the nails.

How strongly Goodhart's Law applies varies. John Cutler shared the Cutler Variation of Goodhart's Law :

"In environments with high psychological safety, trust, and an appreciation for complex sociotechnical systems, when a measure becomes a target, it can remain a good measure because missing the target is treated as a valuable signal for continuous improvement rather than failure."

Related Ideas to Goodhart's Law

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Published 11 Nov 2017

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For want of a nail

If you lived on Easter Island, would you really cut down the last mighty tree knowing that would be it? It seems either foolhardy or desperate to knowingly cut down the last of a kind and your last source of timber. So how could it happen? Easter's End Jared Diamond, in his book Collapse and article Easter’s End, explains how this situation could happen through a gradual, hardly noticeable decline over many years. Those who remembered the great trees that they used to make rafts, canoes, and ships were old or had long since died. Today's trees may be nothing like the forests and towering trunks of tens or hundreds of years ago. As Jared puts it: "Gradually trees became fewer, smaller, and less important. By the time the last fruit-bearing adult palm tree was cut, palms had long since ceased to be of economic significance. That left only smaller and smaller palm saplings to clear each year, along with other bushes and treelets. No one would have noticed the felling of the last small palm." An alternative Easter Island history More recently, I read Rutger Bregman's story of Easter Island in his book Human Kind: A Hopeful History. In the chapter "The Mystery of Easter Island," he digs into the reconstruction provided by Diamond and others and questions whether the Easter Islanders really were reduced, after losing their trees, to infighting and destitution. Bregman found evidence to suggest that rather than driving themselves to the brink of extinction, the Easter Islanders were likely still thriving when Peruvian slave traders arrived in 1862, and sixteen slave ships carried off a third of the island's population, 1,407 people. Yet the worst for them was still to come a year later when the government was pressured to return the islanders home. A smallpox outbreak carried by the returning islanders wreaked devastation during the voyage, with hundreds dying at sea. The 15 islanders who made it home carried the virus with them, and it spread to the inhabitants who remained. He cites that the population was reduced to just 110 inhabitants in 1877 when the epidemic finally subsided. Until the arrival of the slave ships, the population, despite their trials, was still thriving. Bregman suggests that it was, in fact, contact with the rest of the world that destroyed Easter Island. For another thought-provoking perspective, Seth Godin asks who cut down the second-to-last-tree?…If you lived on Easter Island, would you really cut down the last mighty tree knowing that would be it? It seems either foolhardy or desperate to knowingly cut down the last of a kind and your last source of timber. So how could it happen? Easter's End Jared Diamond, in his book Collapse and article Easter’s End, explains how this situation could happen through a gradual, hardly noticeable decline over many years. Those who remembered the great trees that they used to make rafts, canoes, and ships were old or had long since died. Today's trees may be nothing like the forests and towering trunks of tens or hundreds of years ago. As Jared puts it: "Gradually trees became fewer, smaller, and less important. By the time the last fruit-bearing adult palm tree was cut, palms had long since ceased to be of economic significance. That left only smaller and smaller palm saplings to clear each year, along with other bushes and treelets. No one would have noticed the felling of the last small palm." An alternative Easter Island history More recently, I read Rutger Bregman's story of Easter Island in his book Human Kind: A Hopeful History. In the chapter "The Mystery of Easter Island," he digs into the reconstruction provided by Diamond and others and questions whether the Easter Islanders really were reduced, after losing their trees, to infighting and destitution. Bregman found evidence to suggest that rather than driving themselves to the brink of extinction, the Easter Islanders were likely still thriving when Peruvian slave traders arrived in 1862, and sixteen slave ships carried off a third of the island's population, 1,407 people. Yet the worst for them was still to come a year later when the government was pressured to return the islanders home. A smallpox outbreak carried by the returning islanders wreaked devastation during the voyage, with hundreds dying at sea. The 15 islanders who made it home carried the virus with them, and it spread to the inhabitants who remained. He cites that the population was reduced to just 110 inhabitants in 1877 when the epidemic finally subsided. Until the arrival of the slave ships, the population, despite their trials, was still thriving. Bregman suggests that it was, in fact, contact with the rest of the world that destroyed Easter Island. For another thought-provoking perspective, Seth Godin asks who cut down the second-to-last-tree?

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