"Take, for example ... GDP."

"Unfortunately, those untrained in scientific thinking will often mistake the measurement for the thing. This is very common in economics. Take, for example ... GDP.

"GDP is a measure of economic growth, but it is not economic growth in and of itself (nor is it a theory of economic growth).  

"GDP ... is an accounting identity that attempts to act as a proxy for economic growth. But ...those who confuse the measurement for the thing erroneously conclude that an increase in GDP necessarily means an increase in economic growth.  

"GDP is defined as New Consumption + New Investment + New Government Spending + New Net Exports. If any of those variables change, GDP will necessarily rise. That is true. But it does not follow that the rise in GDP necessarily means economic growth is occurring. 

"America in World War 2 and the USSR showed that conclusively. US GDP rose significantly in World War 2 because of the huge increase in government spending. But, by many measures, people were worse off than during the Great Depression: consumer goods were hard to find because so many materials were needed for the war effort, people had to grow their own food—it was not an economy that supports a good life. In the USSR, GDP was rapidly approaching the US. Indeed, some were even predicting the USSR would overtake the US. But once the USSR collapsed and we saw behind the veil, the standard of living for Soviet citizens had barely changed since the fall of the Tsars. GDP was propped up by government spending, and thus became an unreliable indicator of economic growth.

"The confusion between the measurement and thing I have discussed here is a perpetual problem for any sort of central planning or industrial policy. The central planners must establish some goal, which in turn requires some measurement. But then the project becomes all about hitting the measurement rather than promoting the goal. Ultimately, this leads to the plan to fail in its goal even if it hits the measurements."

~ Jon Murphy from his article 'The Measurement is Not the Thing'