The China Economic Review reports:

The size of China's economy was revised downward by 40% in purchasing power parity (PPP) terms in a recent World Bank study of its 2005 GDP, the Wall Street Journal reported. As measured in GDP, China's US$8.8 trillion GDP in 2005 was about 15% of the global total, but under a PPP calculation - which standardizes the buying power across countries with different currencies - found China's economy was recalculated to US$5.3 trillion. The purpose of the study, which included prices of over 1,000 goods in 100 countries, was to get a clearer picture of countries' relative economic strength. The prices analyzed from China came from 11 cities and so did not accounted for prices in rural areas, though the report estimated the margin of error was not more than 5%.

Source:

http://www.chinaeconomicreview.com/dailybriefing/2007_12_18/World_Bank:_Chinas_buying_power_lower_than_expected.html