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
The Penn World Table (PWT) is a set of national-accounts data developed and maintained by scholars at the University of California, Davis and the Groningen Growth Development Centre of the University of Groningen to measure real GDP across countries and over time. Successive updates have added countries (currently 167), years (1950-2014), and data on capital, productivity, employment and population. The current version of the database, version 9, thus allows for comparisons of relative GDP per capita, as a measure of standard of living, the productive capacity of economies and their productivity level. Compared to other databases, such as the World Bank's World Development Indicators, the time period covered is larger and there is more data that is useful for comparing productivity across countries and over time.