The scientific method that is equitable, and can be fairly applied relies on the Purchasing Power Parity (PPP) index produced by the World Bank. The World Bank is a recognized, comprehensive, global source for such data and they generate the most comprehensive and reliable information for this type of measurement.

In its simplest form, the PPP index is a comparison of the average prices of goods and services between two countries. The conversion occurs in a way to ensure that a given amount of one country's currency will purchase the same basket of goods (i.e.: food, clothing, shelter, etc.) and services in the second country as it does in the first.

To understand the PPP index and the information that it conveys, consider a good that remains the same, no matter where the item is sold. For this example, let's assume that a good costs $500 USD in the United States and $600 USD in Country A. Suppose that $600 USD is the equivalent of 3,000 units of country A's local currency (using the official exchange rate established by the World Bank). Then, we could say that same good that costs $500 USD costs 3,000 units of local currency in Country A. Thus, the PPP index would express the relative prices of the same good in each country's currency (3,000/500=6).

The actual PPP measure includes hundreds of goods from approximately 155 categories of GNP, for each country in the study. It also applies weights to depict the relative use of these goods, so that the index represents accurately the importance of these goods to the economy.