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Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced in a specific time period by countries. Due to its complex and subjective nature this measure is often revised before being considered a reliable indicator. 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) may be more useful when comparing living standards between nations, while nominal GDP is more useful comparing national economies on the international market. Total GDP can also be broken down into the contribution of each industry or sector of the economy. The ratio of GDP to the total population of the region is the per capita GDP (also called the Mean Standard of Living).
GDP definitions are maintained by a number of national and international economic organizations. The Organisation for Economic Co-operation and Development (OECD) defines GDP as "an aggregate measure of production equal to the sum of the gross values added of all resident and institutional units engaged in production and services (plus any taxes, and minus any subsidies, on products not included in the value of their outputs)". An IMF publication states that, "GDP measures the monetary value of final goods and services—that are bought by the final user—produced in a country in a given period of time (say a quarter or a year)."
GDP is often used as a metric for international comparisons as well as a broad measure of economic progress. It is often considered to be the "world's most powerful statistical indicator of national development and progress". However, critics of the growth imperative often argue that GDP measures were never intended to measure progress, and leave out key other externalities, such as resource extraction, environmental impact and unpaid domestic work. Critics frequently propose alternative economic models such as doughnut economics which use other measures of success or alternative indicators such as the OECD's Better Life Index as better approaches to measuring the effect of the economy on human development and well being.