Goodhart’s Law

Goodhart’s Law refers to the principle that a measure loses its value the moment it becomes a target.

Usually, when we want to know something, we measure representative values that are easy to measure. For example, when we want to know an employee’s contribution to work, we measure things like working hours or the amount of documents written. However, not everything is easily measurable. Work contribution also includes things that are hard to measure, such as communication skills or how much someone fosters a friendly team atmosphere.

In such situations, when the measured value itself becomes the goal, the measured value becomes meaningless. For example, if an employee’s work contribution is measured by working hours, people will meaninglessly just increase their working hours. They dig into the gap between the measured value and the actual goal, acting only to raise the metric. They become reluctant to engage in behaviors that are important but not measured. As a result, the metric goes up, but the original goal is not achieved—and things actually get worse.

Similarly,

• If rewards are given based on the number of snake heads brought in to eradicate snakes, people instead start snake farms.
• If surgeons are judged by their surgical success rate, they end up refusing to take on risky patients at all.

For this reason, it’s best to use measurements only as reference material for judgment, and let humans make the final decision qualitatively. If qualitative evaluation is difficult, it’s best to judge using a combination of several metrics, without disclosing which metrics are being used.