What law states that when a measure becomes a target, it ceases to be a good measure?

Goodhart's law articulates the idea that once a specific measure is used as a target for decision-making, it can lose its effectiveness as a reliable indicator of the phenomenon it was initially meant to assess. This concept emerges in contexts where the measured variable becomes a goal, leading individuals or organizations to manipulate or game the measure. Consequently, the inherent value or accuracy of the measure diminishes, resulting in unintended consequences.

This principle is particularly relevant in fields such as economics and social sciences, where performance metrics can incentivize behaviors that compromise the original intent of the measurement. For instance, if businesses aim to enhance their performance metrics (like sales numbers), they might prioritize short-term gains over sustainable practices, thereby distorting the measure itself.

The other concepts, such as Parker’s principle, Murphy’s law, and Fisher’s theorem, do not pertain to the relationship between measures and targets in the same way. Parker's principle is not widely recognized in this context, Murphy's law deals with the tendency for things to go wrong, and Fisher's theorem relates to statistics and is primarily focused on the properties of estimators. Thus, Goodhart's law is the most appropriate answer in describing the specific phenomenon where measures lose their integrity when they become targets.