<A href=”http://www.nytimes.com/2015/03/25/business/economy/grading-teachers-by-the-test.html?smprod=nytcore-ipad&smid=nytcore-ipad-share“>Eduardo Porter</a>: <blockquote>The question is, what will happen when teachers are systematically rewarded, or punished, based to some extent on standardized tests? If we really want our children to learn more, the design of any system must be carefully thought through, to avoid sending incentives astray.

“When you put a lot of weight on one measure, people will try to do well on that measure,” Jonah Rockoff of Columbia said. “Some things they do will be good, in line with the objectives. Others will amount to cheating or gaming the system.”

The phenomenon is best known as Goodhart’s Law, after the British economist Charles Goodhart. Luis Garicano at the London School of Economics calls it the Heisenberg Principle of incentive design, after the defining uncertainty of quantum physics: A performance metric is only useful as a performance metric as long as it isn’t used as a performance metric.</blockquote>