These are lists of regions and countries by their estimated real gross domestic product (GDP) in terms of purchasing power parity (PPP), the value of all final goods and services produced within a country/region in a given year. GDP dollar (international dollar) estimates here are derived from PPP estimates. Methodology In the absence of sufficient data for nearly all economies until well into the 19th century, past GDP cannot be calculated, but at best only roughly estimated. In a first step, economic historians try to reconstruct the GDP per capita for a given political or geographical entity from the meagre evidence. This value is then multiplied by estimated population size, another determinant for which as a rule only little ancient data is available. A key notion in the whole process is that of subsistence, the income level which is necessary for sustaining one's life. Since pre-modern societies, by modern standards, were characterized by a very low degree of urbanization and a large majority of people working in the agricultural sector, economic historians prefer to express income in cereal units. To achieve comparability over space and time, these numbers are then converted into monetary units such as International Dollars, a third step which leaves a relatively wide margin of interpretation. The formula thus is: GDP (PPP) = GDP per capita (PPP) x population size It should be stressed that, historically speaking, population size is the far more important multiplier in the equation. This is because, in contrast to industrial economies, the average income ceiling of premodern agrarian societies was quite low everywhere, possibly not higher than twice the subsistence level. Therefore, the total GDP as given below primarily reflects the respective historical population size, and is much less indicative of contemporary living standards than, for example, estimations of past GDP per capita are. According to the 20th-century macroeconomist Paul Bairoch, a pioneer in historical economic analysis, Rather, Bairoch advocates a formula combining GNP per capita and total GNP to give a better measure of the economic performance of national economies. World 1750–1990 (Bairoch) In his 1995 book Economics and World History, economic historian Paul Bairoch gave the following estimates in terms of 1960 US dollars, for GNP from 1750 to 1990, comparing what are today the Third World (Asia, Africa, Latin America) and the First World (Western Europe, Northern America, Japan) A Third World refers to Asia (excluding Japan), Africa, and