Social scientists have long understood that statistical metrics are at their most pernicious when they are being used to control the world, rather than try to understand it. Economists tend to cite their colleague Charles Goodhart, who wrote in 1975: ‘Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes. (Or, more pithily: ‘When a measure becomes a target, it ceases to be a good measure.’) Psychologists turn to Donald T. Campbell, who around the same time explained: “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.
Goodhart and Campbell were on to the same basic problem: a statistical metric may be a pretty decent proxy for something that really matters, but it is almost always a proxy rather than the real thing.
Excerpt from: How to Make the World Add Up: Ten Rules for Thinking Differently About Numbers by Tim Harford