Gross domestic product
Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced in a period of time, often annually 
GDP (nominal) per capita does not, however, reflect differences in the cost of living and the inflation rates of the countries; therefore using a basis of GDP per capita at purchasing power parity (PPP) is arguably more useful when comparing differences in living standards between nations.
Penn World Table
A common practice for comparing GDPs across countries has been to use exchange rates. However, this assumes that this relative price - based on traded products - is representative of all relatives prices in the economy, i.e. that it represents the purchasing power parity (PPP) of each currency. By contrast, PWT uses detailed prices within each country for different expenditure categories, regardless of whether the output is traded internationally (say, computers) or not (say, haircuts). These detailed prices are combined into an overall relative price level, typically referred to as the country's PPP. The detailed prices used to compute PPPs are based on data published by the World Bank as part of the International Comparison Program (ICP).