The Conference Board, a global research association, made a splash with their 2011 global outlook. The group’s most interesting claims are that emerging markets will drive “global growth” and that China could pass the U.S. on one measure of economic size as early as 2012. The Conference Board is making two mistakes many observers make, and which the media gladly eats up.

First, the Conference Board projects China could have a larger economy than America when adjusting for purchasing power parity (ppp). PPP is a way to account for different prices across countries. For example, most things are cheaper in China than the U.S., so a dollar’s worth of money, or 6.7 yuan, generally buys more in the PRC than the U.S.

In that light, the dollar value of China’s GDP should be revised higher in comparison to America’s. For 2009, the World Bank has American GDP near $14.3 trillion and Chinese GDP at $9.1 trillion using ppp, where using normal GDP China was at $4.9 trillion.

Moreover, China almost always revises GDP higher after the fact and boasts much faster growth than the U.S. It’s not going to pass the U.S. in 2012 but, in current ppp terms, it could get close. Hence the headline.

Now the part headlines miss: prices change. What a dollar’s worth of money buys in the PRC is slipping. Chinese prices are rising faster than American prices, arguably much faster. The ppp comparison between the U.S. and China’s is going to change, making China’s economy look smaller.

This has happened before. The last time the World Bank adjusted its ppp measurements, the ostensible size of the Chinese economy fell 40 percent. PPP has advantages but, as you move farther in time from the price measurements that give purchasing power across economies, ppp can tell a very inaccurate story.

The Conference Board might have adjusted for prices changing over time but they gave no indication of having done so. More important: most commentators will not adjust for changing prices; they will take the current ppp measurement and run. That will in turn generate a lot of false claims that China’s economy is soon to be bigger than America’s.

The second mistake the Conference Board made is already common: fast-growing economies drive global growth. That seems sensible but it gives fast-growing economies too much credit. Fast-growing economies may be helping everyone but they may be only helping themselves.

In 2010, China will not add to the rest of the world’s GDP, its trade surplus means it will take almost $200 billion away from the rest of the world’s GDP. This is just a function of how GDP is counted. The PRC does contribute to the world economy in many ways but it is badly misleading to suggest that it is doing the most to help the rest of the world. China is raising the average of GDP growth among countries but doing so in part by continuing to drain GDP from the rest of the world.

In terms of adding to the rest of the world’s GDP, even though we’re growing slowly, the U.S. remains by far the biggest contributor.