Goodhart's law states that when a measure becomes a target, it ceases to be a good measure. In other words, if you pick a measure to assess performance, people find a way to game it. To illustrate, I like the (probably apocryphal) story of a nail factory that sets "Number of nails produced" as its measure of productivity. The workers figure out they can easily make tons of tiny nails to hit the target. Yet, when the frustrated managers switch the assessment to "weight of nails made", the work...
