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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

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

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 their measure of productivity and the workers figure out they can make tons of tiny nails to hit the target easily. 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 trying to measure fitness by steps from a pedometer only to find the pedometer gets 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.

Also see: Campbell's Law

I revised the illustration. Here is the original

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