The thing you measure becomes the thing you optimize.

We tend to think of metrics as a neutral tool for understanding the world. But metrics are not neutral. They are a form of attention. They tell people what matters. And what people believe matters is what they will try to improve.

This is Goodhart’s Law: “When a measure becomes a target, it ceases to be a good measure.” The act of measuring something changes the behavior of the people being measured. They will find a way to make the number go up, even if it means sacrificing the underlying goal.

This is not because people are malicious. It is because they are rational. They are responding to the incentives you have created. If you measure the number of lines of code, you will get more lines of code. If you measure customer satisfaction, you will get more satisfied customers.

What this changes in practice: Choose your metrics with extreme care. For every metric, ask yourself: “What behavior will this incentivize? And is that the behavior I want?”