What does it mean when someone says a city’s economy is taken out of a nation’s economy?

1. Yes they’re effectively subtracting GDP of the City from the national economy (also subtracting the population too if using GDP per capita)

2. There are different ways to capture GDP, and probably different ways to capture this specifically, they will all likely converge on a similar number unless they’re trying to account for that City’s spillivers which would be difficult and probably (intentionally) misleading.

3. Like any metric it can have some uses to make a point. I think we have to be careful with this one specifically because people usually compare the UK minus London to other countries (or US states) as a whole. I think this is unfair. You would also have to take the major city(ies) out of the comparison group in order for it to be a really fair comparison imo. If you do this however it can be a useful way of visualising geographic inequality. Though there are numerical ways of doing that too.

Edit: I want to add a bit more detail and example here because I feel my initial comment was too short

If someone says “If you take away London the UK has a lower GDP per capita than Romania” this is a bad/misleading use of statistics imo. People generally travel to the capital city when they’re young because there’s more opportunity i.e. the average age is significantly lower. By cutting out the capital you are ignoring this dimension and comparing Romania with all of the wealth of its capital to the UK without to give the impression the UK is extremely poor outside of London.

Coversely, someone saying “Take out London from the UK and Berlin from Germany, and the UK’s GDP per capita is much lower than Germany’s” I think is fine for showing that regional inequality is higher in the UK even if the overall GDP per capita is similar