This is called Goodhart's law, who said: "Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes." But

This is called Goodhart's law, who said: "Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes." But in modern usage it is often rephrased to: "When a measure becomes a target, it ceases to be a good measure" https://en.wikipedia.org/wiki/Goodhart%27s_law https://en.wikipedia.org/wiki/Goodhart%27s_law