China’s GDP growth made news this week because, on the official figures, China overtook Japan to become the second largest economy in the world in 2010. But actually, on a different way of calculating the data, this was very old news. Using purchasing power parity, China not only overtook Japan way back in 2001, but it is also quite close to overtaking the US as the biggest economy in the world – if, indeed, it has not done so already.

GDP statistics measure the amount of value added or income in the economy, measured in domestic currencies, over a given period of time. But it is more difficult to compare the GDP in one economy (China) with that in another economy (Japan), because we need to use an exchange rate which translates yuan into yen or vice versa. This is not as straightforward as it may seem.

The simplest method is just to take the market exchange rate over the period in question. This is the basis used in the comparisons which this week showed the level of GDP in China exceeding that in Japan for the first time in 2010.