Goodhart’s Law is one of those neat formulations that codifies something I’ve been trying to put my finger on for years: “once a social or economic indicator or other surrogate measure is made a target for the purpose of conducting social or economic policy, then it will lose the information content that would qualify it to play such a role.”
That is, once you start measuring GDP as a way of gauging social welfare, people will start to figure out ways to make GDP go up without improving social welfare (say, by swapping dirty financial derivatives). Once Google starts measuring inbound links as a way of evaluating the importance of web-pages, people will figure out how to increase the inbound links to unimportant pages (splogging, blogspam). And once you measure fat or calorie content as a proxy for the healthfulness of food, manufacturers will figure out how to decrease fat and calories without making the food more healthful (reducing fat by adding sugar, reducing calories by adding poisonous artificial sweeteners).
Do Boing Boing