When a measure becomes a target
Hospitals cut waiting times by not counting patients. Schools teach to the test. The same law explains both.
2 min read
The economist Charles Goodhart, advising the Bank of England in 1975, noticed that monetary statistics which had been reliable guides stopped behaving once the government began targeting them. The anthropologist Marilyn Strathern later summarised the idea in the sentence everyone quotes.
It is everywhere
- Schools judged by test scores teach to the test.
- Call centres measured by call length hang up on difficult customers.
- Software teams measured by lines of code write longer code.
- Hospitals with waiting-time targets have kept patients in ambulances outside so the clock wouldn’t start.
The psychologist Donald Campbell described the same thing in social policy in the 1970s (“Campbell’s law”): the more a quantitative indicator is used for decisions, the more it gets corrupted.
What to do about it
Goodhart’s law doesn’t mean “never measure”. Some useful habits:
- Use several measures that are hard to game together.
- Keep judgement in the loop. Numbers inform decisions; they shouldn’t make them.
- Rotate or hide the exact metric when possible.
- Ask regularly: is this number still telling us what we think it does?
“When a measure becomes a target, it ceases to be a good measure.”
Bring it up
“There’s an economic law that explains why almost every KPI eventually gets gamed.”
Follow the thread
- RelatedKazakhstan brought half of a dying sea back to life Planners chasing cotton output targets ignored the sea the numbers didn’t measure.
- RelatedBhutan measures Gross National Happiness Any national index risks becoming a target, or a public-relations tool.
- RelatedChesterton’s fence: ask why it’s there before you tear it down Rules and measures often exist for reasons that disappear once you tinker with them.
- RelatedThe Dunning–Kruger effect is not what the internet says Both are about the gap between how we measure ourselves and how things really are.
- Same patternThe cobra effect: when rewards backfire Bounties are the purest example: count corpses, get farms.
- In contrastThe Big Mac index The Big Mac index works partly because nobody is rewarded for moving it.
- RelatedDo skirts really get shorter when the economy booms? A tidy pattern in data is not the same as a cause.
