tl;dr: Comparing the size of two or more economies, e.g., China and the US, is fraught with peril. Traditional comparison tools, such as the Gross Domestic Prod

tl;dr: Comparing the size of two or more economies, e.g., China and the US, is fraught with peril. Traditional comparison tools, such as the Gross Domestic Product (GDP) are neither precise nor accurate and involve considerable nuance. More recent tools such as PPP (Purchasing Power Parity) give the appearance of validity and reliability since they appear to have greater precision - but no one, perhaps least of all the economists using them - can make advance sufficiently meaningful claims as to their accuracy. In other words, PPP may be just as flawed and just as nuanced as GDP. Or it may not. No one really knows. So take with a grain of salt any claims that the Chinese economy is set to overtake the US economy. It may be about to do so. It may not. It may have already done so. It might never do so. Whether or not it does, did, or never will cannot be reliably established with GDP or PPP or any other tools yet known to economists. On a personal note, I'd assert that just as currency strength is a measure of faith in an economy, so goes the importance of whether one economy is perceived as larger than another, with the same vicious/virtuous circle implications: If the Chinese economy is perceived as larger the US and likely to yet larger get, then speculators will put their money in China and remove it from the US, with virtuous and vicious implications for the two economies. Alternatively, if China and the US are perceived as being of a size, then both will for a time rise and fall with the perception of whether or not their being of a size is a good thing or not. In other words, why isn't China growing, let's put our money in the US, both are growing, both are fair picks, both are stagnating, Europe looks good, etc.