Real GDP change for existing goods is usually measured by amount.

But the basket of goods and services produced by an economy changes over time. And it is not necessarily meaningful to compare screws with ball bearings, or integrated circuits with railroad cars. So as a macroeconomic figure, real GDP does not necessarily make sense, even when defined in terms of real goods and services.

This doesn't mean that you can't reasonably parametrise real GDP growth with analytical functions to make an economic theory.

Who promised you that it's an analytical function? For that matter, who promised you that it's a function at all of the variables it's usually expressed in terms of?

(You can always make it a function by parametrising it in time, but as Keynes notes in the paragraphs above, that's not terribly interesting in terms of predictive power...)

- Jake

Friends come and go. Enemies accumulate.