stock-to-flow

Dan Davies comments at Frances Coppola's blog:

Stock-to-flow ratios aren't per se silly or meaningless - return on capital is a ratio of stock to flow, as is labour productivity. But comparing stocks to flows is a dangerous business, because if you don't know what you're doing (as Nelson doesn't), you're always in danger of making the mistake that he has actually made in Chart 1, which is to compare a change in a stock to a flow.

This is a bit of a quibble as the key point here is the one you make - if you're borrowing £2 of "debt" for every £1 of "growth" then you're almost certainly doing very well, because the debt stays at £2 until you pay it back, but the growth gives you a new £1 every year (making the correct stock/flow division would tell you that the UK was getting a 50% return on marginal borrowing, which anyone can see is a pretty fantastic return on investment to be getting. This would still be dumb economics because it's wrongly aggregated, but at least the division would be giving you something at least potentially meaningful).

As I say more of a quibble than anything - I just worried that people might get the impression that comparing a stock with a flow was always wrong, rather than something like starting a sentence with a preposition - probably best avoided but sometimes necessary.