Campbell's law

Campbell's law is an adage developed by Donald T. Campbell, an example of the cobra effect:

"The more any quantitative social indicator is used for social decision-making, the more subject it will be to corruption pressures and the more apt it will be to distort and corrupt the social processes it is intended to monitor." (p. 85)

Campbell's law was published in 1976 by Campbell, a social psychologist, an experimental social science researcher and the author of many works on research methodology: "When a measure becomes a target, it ceases to be a good measure." It is also called Goodhart's law.

The social science principle of Campbell's law is sometimes used to point out the negative consequences of high-stakes testing in U.S. classrooms. This may take the form of teaching to the test or outright cheating. An example is The High-Stakes Education Rule as described in the Learning-Disadvantage Gap.

What Campbell also states in this principle is that "achievement tests may well be valuable indicators of general school achievement under conditions of normal teaching aimed at general competence. But when test scores become the goal of the teaching process, they both lose their value as indicators of educational status and distort the educational process in undesirable ways. (Similar biases of course surround the use of objective tests in courses or as entrance examinations.)"

Closely related ideas are known under different names, e.g. Goodhart's law, and the Lucas critique. Another concept related to Campbell's law emerged in 2006 when UK researchers Rebecca Boden and Debbie Epstein published an analysis of evidence-based policy, a practice espoused by Prime Minister Tony Blair. In the paper, Boden and Epstein described how a government that tries to base its policy on evidence can actually end up producing corrupted data because it, "seeks to capture and control the knowledge producing processes to the point where this type of ‘research’ might best be described as ‘policy-based evidence’." (Boden and Epstein 2006: 226)

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