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.

As well, that Basically I've managed to convince myself that there isn't a sensible mathematical definition of "real GDP" or the "price level" (also known as "GDP deflator")
is not a great intellectual achievment. GDP isn't well defined in infinitesimal small time intervals, or at least it will wiggle extremely around - e.g. on the day scale, GDP at night is much lower than in the morning. Therefore usually the smalles time unit in which GDP is published are months. This doesn't mean that you can't reasonably parametrise real GDP growth with analytical functions to make an economic theory.

If you have two collections of prices and volumes that are exactly the same 10 years apart, you have the same (nominal gdp change) but if the evolution of the economy took two sufficiently disparate paths over those 10 years you could have two very different (real gdp change) and (inflation).

Most economists teach a theoretical framework that has been shown to be fundamentally useless. -- James K. Galbraith