Goodhart's Law - When a measure becomes a target, it ceases to be a good measure

What Does “Goodhart's Law - When a measure becomes a target, it ceases to be a good measure” Mean?

A number is useful because it reflects something you care about. Once people are rewarded or punished for hitting that number, they find ways to move the number without improving the underlying thing - and the measure stops telling you the truth.

Origin and History

Economist Charles Goodhart described the effect in 1975 in a paper on British monetary policy, noting that any observed statistical regularity tends to collapse once pressure is placed on it for control purposes. The popular wording 'when a measure becomes a target, it ceases to be a good measure' is anthropologist Marilyn Strathern's 1997 paraphrase.

Examples

• Customer support: if agents are judged only on tickets closed, tickets get closed without being solved.
• Software: rewarding lines of code produces longer code, not better software.
• Education: when test scores decide funding, teaching narrows to what is tested.
• Health care: waiting-time targets can lead to patients being counted in creative ways rather than seen sooner.

What Happens When You Break It

Ignoring it produces gaming: call-centre agents ending calls early to hit handling-time targets, schools teaching to the test, or teams closing tickets without solving the problem. The dashboard looks green while the real situation gets worse.

Benefits of Following This Rule

Keeping Goodhart's Law in mind leads to better incentive design: using several balanced measures, rotating or auditing metrics, pairing numbers with qualitative review, and remembering what the metric was supposed to represent.