The moment a measure becomes a target, it stops measuring anything useful

Economist Charles Goodhart observed that any statistical regularity used as a policy target tends to break down, because people start optimising for the number itself rather than what it was meant to reflect. Call centres that reward call volume get rushed calls; schools that reward test scores get teaching to the test. The measure gets gamed precisely because it now matters.

— Charles Goodhart, Problems of Monetary Management: The U.K. Experience — 1975