美国的经济规模在1982年超过英国后一直处于世界第一的位置，但是他的王朝可能就要结束了。根据一项跨国GDP对比，中国现在是世界上最大的制造商。但是，另一种方式的比较，美国的经济规模还远远大于中国。比较哪个经济体规模更大的最好的方法完全取决于一个人选择的方法和比较的动机。

 

跨国比较的问题

GDP测量在给定时间内一个国家产生的产品和服务的货币价值。这里的关键词是货币，其价值本身是不可见的。人们只能观察产品和服务交易时的价格。

 

在试图比较以美元计价的美国商品和服务和以人民币计价的中国商品和服务时变得很明显，这是两种不同单位的测量价值。除非人们了解两种货币的转换因子，不然两种货币的比较是毫无意义的。幸运的是，我们确实有转换因子，不幸的是，我们事实上拥有的两个转换因子存在巨大分歧。

 

这两个转换因子是市场汇率和购买力平价率（PPP）。根据世界银行的数据，2014年中国以市场汇率测定的GDP为10.36万亿美元，而美国为17.42万亿美元。然而，世界银行根据购买力平价测定中国GDP为18.03万亿美元，美国仍然是17.42万亿美元。这其中存在很大差距。

 

市场汇率和购买力平价对比

市场汇率是使用时间段最后的市场外汇率或者该时间段平均汇率。而购买力平价汇率相反，使用把一种货币购买和使用另一种货币购买相同数量的产品和服务的转换率。

 

虽然市场汇率看起来非常简单，但是我们来看一个例子更好的解释购买力平价测量。假设在美国买一个汉堡要平均花费5美元，在中国则花费17元人民币。这意味着美元和人民币的购买力平价转换率为17/5=3.4，因为这将有效的使汉堡在两国的价格相同。

 

然而，跨国GDP比较根据单一产品的价格是远远不够的。需要考虑大量的产品和服务，这也是国际对比项目（ICP）尝试做的事情。ICP发展了一个包含超过1000种商品和服务的全球核心列表。

 

优点和缺点

PPP变成一种重要方式的原因之一是市场汇率方式把GDP转换到同一种货币，而忽视了不同国家货币的相对购买能力。虽然市场汇率方式计算国际间商品和服务的购买能力，它忽视了许多商品在一个国家比另一个国国家便宜很多的事实，这在像中国这样的发展中国家尤为真实。

 

PPP的另一个优点是相对稳定，而市场汇率波动比较大。即使真实增长率比较稳定，测量的增长也会随着市场汇率波动。

 

市场汇率的缺点在阐述PPP优点的时候我们已经说明。然而，市场汇率的一个优点是比PPP容易计算。

 

总结

在试图确定哪一种GDP测量方式在比较国家的经济规模最好时，重要的是要明白你为什么要做比较。如果你的目标是测量生活水平，那就使用PPP。如果你对地缘政治更关心，那么市场汇率就会更适合你。以中国和美国哪个国家的经济规模更大是个好标题，但是如果你不知道GDP测量要做什么则它根本无关紧要。

 

Which Economy Is Larger - The United States or China? 

 

By Matthew Johnston | October 12, 2015 

| 

The U.S. economy has been the world’s largest since overtaking the U.K. economy in 1872, but its reign may just be over. According to one method of making inter-country GDP comparisons, China is now the world’s biggest producer. (See related: The 3 Industries Driving China's Economy.) Yet, according to another method, the U.S. economy is still significantly larger than China’s. The best method, and hence which economy is larger, all depends on one’s methodology and motivations for making the comparison in the first place.

 

Inter-Country Comparison Problems

 

The GDP measures the monetary value of all final goods and services produced in a country over a given period of time. The key word here is monetary, since value itself is unobservable. All one can observe are the prices (i.e. monetized values) at which goods and services exchange. So far so good.

 

Problems soon become evident when trying to compare U.S. goods and services priced in dollars with Chinese goods and services priced in yuan, which are two different units to measure value. Unless one knows the conversion factor, comparisons between two separate currency units are meaningless. Fortunately, we do have a conversion factor; somewhat unfortunately, we actually have two conversion factors that can have large discrepancies.

 

The two conversion factors at our disposal are market exchange rates and purchasing power parity (PPP) rates. According to the World Bank, at market exchange rates, China’s GDP for 2014 was $10.36 trillion, compared to the U.S. figure of $17.42 trillion. However, the World Bank’s figures at PPP rates reveal that China’s GDP for 2014 was $18.03 trillion, while the U.S. figure is still the same $17.42 trillion. That’s a significant difference.

 

Market Exchange Rates vs. Purchasing Power Parity

 

The market exchange rate measure uses the prevailing rate in the foreign exchange market either at the end of the period in question, or averaged over that period. The PPP exchange rate measure, contrastingly, uses the rate at which one currency would need to be converted to another currency in order to buy an equivalent amount of goods and services in each country.

 

While the market exchange rate measure seems simple enough, let’s look at an example to better illustrate the PPP measure. Suppose that it costs an average of $5 to buy a hamburger in the U.S., while it costs an average of 17 yuan to buy one in China. That means the PPP exchange rate of dollars to yuan should be 17/5 = 3.4, since that would be the rate that would effectively make hamburgers the same price in each country.

 

However, basing inter-country GDP comparisons on the relative prices of a single good in separate countries is inadequate. It is important to consider a broad range of goods and services, which is exactly what the International Comparisons Program (ICP) attempts to do. The ICP has developed a Global Core List of over 1,000 goods and services comparable across different regions and priced by all participating countries.

 

Advantages and Disadvantages

 

One of the reasons that PPP became an important method was that where the market exchange rate method converts GDP to a common currency, it fails to take into account the relative purchasing power of each currency in its respective country. While the market exchange rate method accounts for the purchasing power of internationally traded goods and services, it fails to reflect the fact that many goods and services are relatively cheaper in some countries than others; this is particularly true of developing countries like China.

 

Another advantage of PPP rates is that they tend to be more stable over time, whereas market exchange rates are much more volatile. Aggregate measures of growth will fluctuate with the same volatility as exchange rates even when actual growth rates are relatively stable.

 

The disadvantages of the market exchange rate method have been noted already by highlighting the relative advantages of the PPP method. However, one advantage of market exchange rates over PPP rates is the relative ease with which one can do the calculations. Because there is so much more involved in the PPP calculations (i.e. choosing the appropriate basket of goods and services and assigning appropriate weights to each), it is much easier for inaccuracies to be introduced.

 

The Bottom Line

 

In trying to determine which GDP measurement method is best for comparing the relative size of different economies, it is important to understand why you are making the comparison in the first place. If your goal is to use GDP as a measure to compare living standards, then using the PPP method may be your best option as it more adequately measures the relative purchasing power of each currency within its respective country. However, if your concerns are geopolitical, then the exchange rate method is best as it is better at measuring the relative importance of a country’s trading partners. The question of whether the Chinese economy is bigger than the U.S. economy may make nice headlines, but it matters little if one is unsure of why GDP measurements matter to begin with. (See also: China's Economy: Transition to Sustainable Growth.)

 

本文翻译由兄弟财经提供

文章来源：http://www.investopedia.com/articles/investing/101215/which-economy-larger-united-states-or-china.asp