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"The more any quantitative social indicator (or even some qualitative 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."
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.)"
Campbell's law was published in 1976 by Donald T. Campbell, a social psychologist, an experimental social science researcher and the author of many works on research methodology. 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)