What does Godhart’s law mean?

Question

It goes “When a measure becomes a target, it ceases to be a good measure.” How does that work in practice?

In: Other

Answer 1

Exams in schools are basically the absolute perfect example of this.

So what is the thing that we actually want to measure in school? We want to measure how much students learn in school. This is very hard to measure directly, we can’t look into peoples minds and figure it out.

So instead we invent exams and tests. This is our measure. There is a difference however between learning subjects in school and performing well in tests for those subjects so it should be fine right? There is a strong correlation between learning and writing good tests so the test is a good measure at first glance. Students who learned more (for whatever reason) should perform better on tests

But then we reward students for being good at tests (and teachers for their students performing good at tests etc). So now our students try to get good results on exams. The tests and exams have become a target.

All of the small difference between performing well in tests and actually learning become much more pronounced because people try to find methods to get the desired results on these tests and many of these might not have any direct influence on actually learning the subjects. The tests have become a way worse measure of how much students learn.