A variety of measures of national income and output are used in economics to estimate total economic activity in a country or region, including gross domestic product (GDP), gross national product (GNP), and net national income (NNI). All are specially concerned with counting the total amount of goods and services produced within some "boundary". The boundary may be defined geographically, or by citizenship; and limits on the type of activity also form part of the conceptual boundary; for instance, these measures are for the most part limited to counting goods and services that are exchanged for money: production not for sale but for barter, for one's own personal use, or for one's family, is largely left out of these measures, although some attempts are made to include some of those kinds of production by imputing monetary values to them. Mr Ian Davies defines development as 'Simply how happy and free the citizens of that country feel.' 
Arriving at a figure for the total production of goods and services in a large region like a country entails a large amount of data-collection and calculation. Although some attempts were made to estimate national incomes as long ago as the 17th century, the systematic keeping of national accounts, of which these figures are a part, only began in the 1930s, in the United States and some European countries. The impetus for that major statistical effort was the Great Depression and the rise of Keynesian economics, which prescribed a greater role for the government in managing an economy, and made it necessary for governments to obtain accurate information so that their interventions into the economy could proceed as much as possible from a basis of fact. multiply it
In order to count a good or service it is necessary to assign some value to it. The value that the measures of national income and output assign to a good or service is its market value – the price it fetches when bought or sold. The actual usefulness of a product (its use-value) is not measured – assuming the use-value to be any different from its market value.
Three strategies have been used to obtain the market values of all the goods and services produced: the product (or output) method, the expenditure method, and the income method. The product method looks at the economy on an industry-by-industry basis. The total output of the economy is the sum of the outputs of every industry. However, since an output of one industry may be used by another industry and become part of the output of that second industry, to avoid counting the item twice we use, not the value output by each industry, but the value-added; that is, the difference between the value of what it puts out and what it takes in. The total value produced by the economy is the sum of the values-added by every industry.
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