Looking at what goes into GDP is usually a pretty tedious affair, but the simplest way to think of it is like this: GDP is meant to represent the total value added. It is new work done; new stuff produced.
One upshot of this is that new houses are counted in GDP, while sales of existing houses are not. This is because sales of existing houses are just value transferred – an exchange of assets – and so don’t represent new effort. That’s not quite true. The real-estate agent fees and legal fees associated with the sale count, since they are new work done: they add new value by facilitating the trade.
Here’s a trick in looking at value added: we only need to look at the prices of final goods. This is because the price of the final good will represent the total value added along the entire production chain. The typical example of this used in introductory textbooks is bread:
|Supermarket||Packaged and convenient bread||$1.00||$0.55|
The price of the final good – packaged, convenient bread – is $1.00, which exactly equal to the sum of all the value added. So when the statisticians want to calculate a country’s GDP, they can ignore all the intermediate levels and just add up all the final goods that were produced.So what counts as a final good? Anything that gets sold to someone for consumption or investment. That might be to an individual, or to a private firm, or the government, or someone overseas. (Of course, since I buy both bread and flour from my supermarket, flour is sometimes an intermediate good and sometimes a final good; but it’s easy to tell which is which – flour sold by the supermarket is final, while flour sold by the miller is intermediate.)
Now consider a country that has a large natural resource sector. Australia is a great example. So are all the oil exporting countries. We’ll pick the mining of iron ore in Australia as an example. Just like with the wheat above, there is a whole range of production possibilities based on the iron ore. However, when it’s exported, the final good that gets counted from the point of view of the Australian economy is the iron ore in the ship as it sails off to another country.The mining companies are definitely adding value. They’ve got to find the stuff in the first place, dig it up, clean it a bit to get rid of the dirt, transport it to the coast and then ship it overseas. They’ve also got to maintain all their equipment and allow for the fact that they wear out over time. All of that is new effort. But the price that India or China pays for the ore is more than cost of doing all of that. A large fraction of the price they pay represents the market value of the underlying asset – the ore – itself. But since the mining company didn’t actually produce the ore, that part of the price shouldn’t really count in GDP, for the same reason that when existing houses are sold, only the agent and legal fees are counted. None of this is really news.
When natural-resource-based industries are only a small part of a country’s economy, there’s not too much distortion, so we tend not to worry about it. But when those industries represent a large share of the national income, then the overestimates can be significant. In Australia, mining represents  about 6.7% of the national economy. A fair chunk of that will be “true” value added, but a large share of it is really just the transfer of assets. How much? Well, BHP currently has a Return on Equity  of 49%, while the long-run, risk-free return on capital is more like 8-10%. So as a very rough guess, assuming that BHP is representative of the mining industry as a whole and that the mining industry is competitive, we might suggest that Australia’s “true” GDP is at least 39% * 6.7% = 2.6% smaller than we think it is.
Some people might at this point wonder about the farmer back in the bread example. What if the farmer who, like BHP, is taking something from the land, is actually only adding 60% of the value that we think she is? The answer lies in the fact that there is a large production chain that builds up from the farmer’s wheat. Even if we remove a large fraction of the farmer’s value-added, that is only a small share of the total value added that we see in the final good’s price. So we would expect this overestimate to be very small overall. The point about mining is that we are only adding a small amount of value relative to that of the asset we are trading away, so as a percentage of the final good, the asset itself is quite large.