The moment you turn a measure into a target, it starts to lie.
In 1975 the economist Charles Goodhart noticed that any statistic used to steer the economy tends to stop behaving once people are pushed to hit it. His point has escaped economics entirely. Hospitals, schools, sales teams and scientists all show the same pattern: people learn to game the number.
Charles Goodhart made his observation in a 1975 article on British monetary policy. His original wording was dry: any observed statistical regularity tends to collapse once pressure is placed on it for control purposes. The punchier version most people know, 'when a measure becomes a target, it ceases to be a good measure', came later, through the accountant Keith Hoskin and the anthropologist Marilyn Strathern in the 1990s.
The idea was not entirely new. A similar principle known as Campbell's law has versions dating back to 1969, and the Lucas critique of 1976 made a related argument about economic policy. Critics used Goodhart's insight against the Thatcher government's attempts to steer the economy by targeting measures of the money supply.
Once you know the pattern, you see it everywhere. A salesperson paid by cars sold each month may sell cars at a loss. Hospitals pushed to cut the average length of stay can discharge patients too early, and readmissions rise. As the h-index became a standard way of judging scientists, its link to major scientific awards weakened. Strathern's own example was university grades: the more a particular degree class becomes an expectation, the less it tells you about any individual student.
The mechanism is simple. A number is useful because it tracks something you care about but cannot see directly. Reward people for the number and they find cheaper ways to move it than improving the real thing. It does not mean measurement is useless, only that a single target invites people to game it, and the best defences are several measures, human judgement, and remembering what the number was for.
Source: Wikipedia — Goodhart's law · Text summarised from Wikipedia (CC BY-SA 4.0)
