Goodhart's law

Goodhart's law is the principle that a measure can become less informative when it is used as a target, because incentives change behavior around it.

Campbell's law: The principle that greater use of a quantitative indicator for decision-making increases pressure to corrupt it and distort the process it monitors. It describes the same feedback between indicator-based decisions and distorted behavior in social programs.
Teaching to the test: An instructional practice that focuses classroom teaching on material and skills likely to appear on standardized tests. Test scores can rise while broader learning receives less attention.
Lucas critique: The argument that policy effects estimated from past behavior may fail when policy changes the incentives shaping that behavior. It shares the insight that people adapt to policy, but focuses on economic forecasts and policy rules.
Charles Goodhart: A British economist whose work on monetary policy gave rise to Goodhart's law. His 1975 formulation connected attempts to control monetary aggregates with their changing statistical behavior.
Metric gaming: The manipulation of measured performance to improve reported scores without proportionate improvement in the underlying goal. Gaming is one direct way people respond when a measure determines rewards or penalties.
Hawthorne effect: A change in behavior that occurs when people know they are being observed or studied. Observation can alter the behavior being measured, even before a formal target is imposed.
Surrogate endpoint: A measurable substitute outcome used in research or trials to predict an outcome that matters directly to patients. A surrogate can fail to predict the real outcome even without becoming an explicit performance target.
Goodhart's 1975 law: Charles Goodhart's observation that statistical regularities tend to break down when used for control purposes. This original formulation concerned monetary policy, before the principle broadened to performance measures.
Proxy variable: A measurable variable used to stand in for a quantity that is difficult or impossible to observe directly. Targets often begin as proxies for outcomes that cannot be measured directly.
Waiting-time target: A performance standard that sets a maximum time patients should wait for specified healthcare services. Strict waiting-time targets have sometimes shifted queues rather than reduced total delays.

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