# goodhart-law-mesure-cible-wikipedia-1975

## Veille

Encyclopedic article (Wikipedia, English) on **Goodhart's law**: stated by British economist Charles Goodhart in 1975 regarding monetary policy — "any observed statistical regularity tends to collapse once pressure is placed upon it for control purposes" — then generalized by anthropologist Marilyn Strathern (1997) into the canonical aphorism "when a measure becomes a target, it ceases to be a good measure." The subject connects economics, incentive theory, public policy evaluation and, by extension, metric optimization in AI systems.

## Titre Article

Goodhart's law

## Date

1975-12

## URL

https://en.wikipedia.org/wiki/Goodhart%27s_law

## Keywords

Goodhart's law, measure becoming a target, statistical regularity, monetary policy, perverse incentives, corrupted indicators, reward hacking, metric optimization, Campbell's law, Lucas critique, cobra effect, McNamara fallacy, accountability, evaluation, h-index, No Child Left Behind, COVID testing targets, gaming of indicators

## Authors

Wikipedia contributors (concept : Charles Goodhart ; généralisation : Marilyn Strathern)

## Ton

Profile: collaborative encyclopedic article (Wikipedia), impersonal third-person perspective, neutral and synthetic academic register, medium technical level accessible to an educated non-specialist audience, target audience students, researchers, policymakers and anyone seeking a reliable, sourced definition. The tone is one of rigorous popularization: successive dated and attributed definitions, history of the formulations, conceptual distinctions from related laws (Campbell, Lucas), then a catalog of sectoral examples (healthcare, research, conservation, education, pandemic). Authority does not come from a single author but from the critical apparatus (sourced quotations, bibliographic references, attribution of authorship). Definitional and enumerative style, without metaphor or persuasive rhetoric; the value lies in the precision of the attributions (who said what, when) and in relating related concepts to one another. No thesis is defended: the article presents an established consensus on a principle widely recognized in the social sciences.

## Pense-betes

- **Original statement (Charles Goodhart, 1975)**: *"Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes"* — formulated in an article on monetary management in the United Kingdom. The economic regularity exploited as a policy lever ceases to hold once it is used as such.
- **Canonical reformulation (Marilyn Strathern, 1997)**: *"When a measure becomes a target, it ceases to be a good measure"* — the generalized and popularized version, from a text on audit in the British university system. It is the most frequently cited formula today, often wrongly attributed to Goodhart himself.
- **Core mechanism**: rational actors optimize *toward the measure* rather than toward the real objective it is supposed to capture → the link between indicator and reality degrades. The metric loses its diagnostic value by becoming an explicit objective.
- **Campbell's law (Donald T. Campbell, 1976/1979)**: a variant centered on quantitative social indicators — the more an indicator is used for social decision-making, the more it is subject to corruption and the more it distorts the processes it was meant to monitor.
- **Lucas critique (Robert Lucas, 1976)**: a macroeconomic relative — the effects of a policy cannot be predicted from observed historical relationships, because agents adapt their behavior to the policy itself.
- **Related concepts**: the **cobra effect** (an incentive inadvertently rewards counterproductive behavior), the **McNamara fallacy** (dismissing what cannot be quantified as irrelevant).
- **Intellectual contributors cited**: Jerome Ravetz (1971, manipulation of systems with complex objectives), Keith Hoskin (1996, linkage to accountability frameworks), Jon Danielsson (application to financial risk modeling).
- **Sectoral examples**: healthcare (reducing length of stay as a target → premature discharges and readmissions); research (the **h-index** loses value once it becomes a target); conservation (IUCN extinction classifications tightened after being used to lift protections); education (**No Child Left Behind** → grade advancement without genuine mastery); pandemic (UK **COVID testing** targets conflating capacity with diagnostic usefulness).
- **Historical root**: a consequence of the modernist accountability practices born in 19th-century Britain.
- **AI relevance (link to draw)**: Goodhart's law is the direct conceptual ancestor of **reward hacking** and of the critique of gameable **coverage gates** / metrics "at machine speed" — cf. Chris Williams' ADLC series (mutation testing preferred over percentage coverage precisely because it is *Goodhart-able*).

## RésuméDe400mots

Goodhart's law is a social-science adage describing how a measure loses its reliability as soon as it is turned into a steering objective. It owes its name to British economist **Charles Goodhart**, who formulated its core in a **1975 article** devoted to monetary policy in the United Kingdom: *"any observed statistical regularity tends to collapse once pressure is placed upon it for control purposes"*. The intuition arose from analysis of British monetary-management difficulties: the stable correlations exploited by central banks as policy levers ceased to hold once instrumentalized.

The most widespread formulation, however, is not Goodhart's but that of anthropologist **Marilyn Strathern**, who in **1997**, in a text on accountability in the university system, proposed the generalized and memorable version: *"when a measure becomes a target, it ceases to be a good measure"*. This reformulation emphasizes the loss of diagnostic value a metric suffers when individuals optimize toward the measure itself rather than toward the underlying objective it is meant to represent.

The idea belongs to a constellation of related principles. **Campbell's law** (Donald T. Campbell, 1976) addresses the corruption of quantitative social indicators used for decision-making. The **Lucas critique** (1976) offers its macroeconomic equivalent: the effects of a policy cannot be predicted from historical relationships, because agents adapt to them. To these are added the **cobra effect** (an incentive that inadvertently rewards counterproductive behavior) and the **McNamara fallacy** (dismissing the qualitative because it escapes quantification). Several authors have enriched the corpus: Jerome Ravetz (1971), Keith Hoskin (1996), and Jon Danielsson for financial risk modeling.

Illustrations span numerous fields: in healthcare, making length of stay a target causes premature discharges and readmissions; in research, the h-index erodes as a measure of reputation as it becomes an evaluation criterion; in conservation, IUCN extinction classifications have been tightened after being used to lift protections; in education, No Child Left Behind encouraged grade advancement without mastery; during the pandemic, UK COVID testing targets conflated capacity with diagnostic usefulness. The principle ultimately reflects how rational actors optimize within measured systems — a legacy of accountability practices born in the 19th century. Today, it directly illuminates *reward hacking* and the fragility of optimization metrics in AI systems.

## GrapheDeConnaissance

- Charles Goodhart —a_créé→ loi de Goodhart (CONCEPT, 0.98)
- Charles Goodhart —affirme_que→ toute régularité statistique observée s'effondre dès qu'on exerce une pression sur elle à des fins de contrôle (CITATION, 0.97)
- loi de Goodhart —observé_dans→ politique monétaire du Royaume-Uni (CONCEPT, 0.92)
- Marilyn Strathern —affine→ loi de Goodhart (CONCEPT, 0.95)
- Marilyn Strathern —affirme_que→ quand une mesure devient une cible, elle cesse d'être une bonne mesure (CITATION, 0.96)
- loi de Goodhart —affirme_que→ un acteur rationnel optimise vers la mesure et dégrade le lien indicateur-réalité (AFFIRMATION, 0.9)
- loi de Campbell —converge_avec→ loi de Goodhart (CONCEPT, 0.88)
- critique de Lucas —converge_avec→ loi de Goodhart (CONCEPT, 0.85)
- Donald T. Campbell —a_créé→ loi de Campbell (CONCEPT, 0.9)
- Robert Lucas —a_créé→ critique de Lucas (CONCEPT, 0.9)
- effet cobra —est_instance_de→ incitation perverse (CONCEPT, 0.85)
- loi de Goodhart —s_applique_à→ h-index (CONCEPT, 0.88)
- loi de Goodhart —s_applique_à→ objectifs de tests COVID au Royaume-Uni (CONCEPT, 0.87)
- No Child Left Behind —observé_dans→ passages de classe sans maîtrise réelle (CONCEPT, 0.84)
- Jon Danielsson —s_applique_à→ modélisation du risque financier (CONCEPT, 0.82)

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Canonical: https://www.thekb.eu/en/fiches/goodhart-law-mesure-cible-wikipedia-1975/