What do the price of hair-salon services in Beijing and sexual services in London have in common? The answer is that, depending on how you measure them – or indeed whether you measure them at all – the size of the Chinese and British economies will expand or contract like an accordion.

In April, statisticians working under the aegis of the World Bank determined that China’s gross domestic product was far bigger than they had previously realised. China was, in fact, just about to overtake the US as the world’s largest economy, many years earlier than expected. The reason? Statisticians had been overestimating the prices of everything from haircuts to noodles. As a result, they were underestimating the purchasing power of Chinese people and thus the size of the economy.

Last month, British statisticians worked some magic too. They declared that the UK economy – admittedly only a fraction of China’s size – was 5 per cent bigger than previously thought. It was as if they had suddenly discovered billions of pounds in annual revenue at the back of the nation’s couch. Here the explanation was simpler. Among other tweaks to their methodology, statisticians started counting the economic “contribution” of prostitution and illegal drugs.