9.5
Goodhart's Law
The principle that an observed statistical regularity collapses once it is used as a target for control.
Goodhart (1975); the famous phrasing is Strathern (1997), citing Hoskin (1996)
What it does
Goodhart's actual wording: “Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes.” The popular version — when a measure becomes a target it ceases to be a good measure — is Strathern's, not his. Either way it is the standing warning over every other entry here.
When it breaks
It does not break. What breaks is the measurement system that forgot it.
Case
The origin case is UK monetary targeting in the 1970s and 80s: once the Bank of England set targets on specific money-supply aggregates, the historical relationship between those aggregates and inflation broke down and the targets were abandoned. Goodhart drew the law from watching it happen.
Goodhart (1975) — Problems of Monetary Management ↗Diagram — not yet drawn
A tight correlation between measure and outcome, then a target line drawn on the measure, then the same scatter afterwards with the relationship gone.
In the wild
Unvetted · not part of the tier assessment
What has been written about this tool in the last twelve months. Machine-retrieved and unchecked — everything above this line was checked.
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