Goodhart's law
When a measure becomes a target, it stops being a good measure. Why that matters, and how to avoid it.
Numbers are useful. We measure things to see whether we are improving. But there is a trap in measurement, and it has a name: Goodhart's law.
The usual phrasing is: when a measure becomes a target, it ceases to be a good measure. It is named after the British economist Charles Goodhart, who made a related point about monetary policy in 1975. The anthropologist Marilyn Strathern later gave it the crisp wording above.
What goes wrong#
A measure is a stand-in for something we care about. Test scores stand in for learning. Sales calls stand in for sales. Lines of code stand in for progress.
Once people are judged or rewarded by the stand-in, they start to optimise it directly. Teachers teach to the test. Staff make many short calls. Programmers write more code than needed. The number goes up, but the thing it was meant to represent may not.
A simple story#
A shop owner wants more visitors and decides to count the number of people who walk through the door. Staff begin handing out free sweets in the street. Visitors rise. Sales do not.
How to avoid it#
- Use several measures. A mix of numbers is harder to game than one.
- Look at the real goal. Ask whether the number is still tracking what you care about.
- Combine numbers with judgement. A human who understands the work can spot when a figure is misleading.
- Change measures from time to time. Old ones lose their meaning once people learn to target them.
In your own life#
This applies to personal goals too. If you aim at 10,000 steps, you might pace around the kitchen at midnight. If you aim for a word count, you might pad your writing. The measure should serve the goal, and not the other way round.
Keep asking: what am I really trying to do? The number is only a guide.