Few people noticed, but China got smaller the other day. According to new estimates, the colossal Chinese economy that has been making marketers salivate and giving others an inferiority complex may be roughly 40 percent smaller than previously thought: worth $6 trillion rather than $10 trillion. That means it lost a chunk roughly the size of Japan’s output.

What happened was a large statistical glitch. When comparing the size of economies, economists mostly avoid using the standard currency exchange rates seen in bank windows. These fluctuate too much, driven by housing woes, trade deficits or presidential popularity. Economists prefer to use what is known as “purchasing power parity”  or P.P.P.  a rate that adjusts for price differences between countries.

Take a 40 yuan serving of noodles at an eatery in Beijing. If the same dish cost $4 at a comparable restaurant in New York, the noodle P.P.P. would be 10 yuan to the dollar. Calculated using a large basket of goods and services, this ratio allows for a more consistent comparison of economies.

The problem is that the World Bank’s measure of China’s rate, everybody’s benchmark, had been based on a 1980s survey of Chinese prices. This year, the World Bank did its own survey to update the measure. While the bank has not published it yet, Albert Keidel of the Carnegie Endowment for International Peace extrapolated the figure from another set of exchange rates published by the Asian Development Bank.