What drives the A$?

Exchange rate forecasting, as we all know, is usually a decidedly iffy proposition: the authors of this new Discussion Paper from the Reserve Bank of Australia point to "the well-documented difficulties in empirically explaining movements in exchange rates" and "the imprecise nature of exchange rate modelling, which is well established in the literature".

I'll just pause for a sec (before I get anyone into trouble) to point out that in any Discussion Paper, "Views expressed in this paper are those of the authors and not necessarily those of the Reserve Bank. Use of any results from this paper should clearly attribute the work to the authors and not to the Reserve Bank of Australia".

Right. Carrying on, and despite the well-known difficulties, they've done a pretty good job of modelling the behaviour of the (real) trade-weighted index (TWI) of the Aussie dollar. Here's how their model fits the data: if I'd managed that, I think I'd be retiring to the pub for a beer after a good day's work.

It's an error-correction model, where the TWI tries to move towards an equilibrium level determined in this model mostly by Australia's terms of trade, with a smaller supporting role for a real interest rate differential ("the real policy rate differential between Australia and G3 economies"). And it explains about half of the quarterly changes in the TWI over 1986-2014.

The authors were a bit exercised by those periods where the actual A$ TWI was well away from the modelled band - below, during the GFC, and above, more recently - and they've had a go at seeing whether various ways of modelling the impact of the recent Australian resource investment boom and of unconventional monetary policy overseas would explain those deviations. There were some suggestive hints, but no knock-out discoveries: "Taken as a whole, while the results from these augmented models support the notion that there have been some additional influences on the real exchange rate in recent years, they do not fully account for the behaviour of the exchange rate during the period". The existing model did more or less as well (and more simply) than potential alternatives.

Incidentally, if you're a student, or hence or otherwise would like to get up to speed with where the economics of exchange rates has got to in recent years, there's a very useful bibliography at the end of the paper.

You're probably wondering, is there a Kiwi dollar equivalent? And yes there is, give or take (the Aussie graph shows the modelled A$ TWI versus actual, the Kiwi graph shows an explanation of why the actual rate is away from its long-term average). Here's what it looks like, and I wrote it up in more detail here.

Takeaways? Two main ones. The story that the A$ and NZ$ are 'commodity backed' currencies is oversimplified, but not wrong. And exchange rate forecasting may be problematic, but I'd say not so problematic that you can't get something useful out of it.