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Overview of the measures of national income and output
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), net national income (NNI), and adjusted national income (NNI adjusted for natural resource depletion – also called as NNI at factor cost). All are specially concerned with counting the total amount of goods and services produced within the economy and by various sectors. The boundary is usually defined by geography or citizenship, and it is also defined as the total income of the nation and also restrict the goods and services that are counted. For instance, some measures count only goods & services that are exchanged for money, excluding bartered goods, while other measures may attempt to include bartered goods by imputing monetary values to them.

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 well informed as possible.

In order to count a good or service, it is necessary to assign 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. The expenditure method is based on the idea that all products are bought by somebody or some organisation. Therefore, we sum up the total amount of money people and organisations spend in buying things. This amount must equal the value of everything produced. Usually, expenditures by private individuals, expenditures by businesses, and expenditures by government are calculated separately and then summed to give the total expenditure. Also, a correction term must be introduced to account for imports and exports outside the boundary. The income method works by summing the incomes of all producers within the boundary. Since what they are paid is just the market value of their product, their total income must be the total value of the product. Wages, proprietor's incomes, and corporate profits are the major subdivisions of income.

GDP per capita (per person) is often used as a measure of a person's welfare. Countries with higher GDP may be more likely to also score high on other measures of welfare, such as life expectancy. However, there are serious limitations to the usefulness of GDP as a measure of welfare:
Measures of GDP typically exclude unpaid economic activity, most importantly domestic work such as childcare. This leads to distortions; for example, a paid nanny's income contributes to GDP, but an unpaid parent's time spent caring for children will not, even though they are both carrying out the same economic activity. GDP takes no account of the inputs used to produce the output. For example, if everyone worked for twice the number of hours, then GDP might roughly double, but this does not necessarily mean that workers are better off as they would have less leisure time. Similarly, the impact of economic activity on the environment is not measured in calculating GDP. Comparison of GDP from one country to another may be distorted by movements in exchange rates. Measuring national income at purchasing power parity may overcome this problem at the risk of overvaluing basic goods and services, for example subsistence farming. GDP does not measure factors that affect quality of life, such as the quality of the environment (as distinct from the input value) and security from crime. This leads to distortions for example, spending on cleaning up an oil spill is included in GDP, but the negative impact of the spill on well being (e. g. loss of clean beaches) is not measured. GDP is the mean (average) wealth rather than median (middle point) wealth. Countries with a skewed income distribution may have a relatively high per capita GDP while the majority of its citizens have a relatively low level of income, due to concentration of wealth in the hands of a small fraction of the population. See Gini coefficient. Because of this, other measures of welfare such as the Human Development Index (HDI), Index of Sustainable Economic Welfare (ISEW), Genuine Progress Indicator (GPI), gross national happiness (GNH), and sustainable national income (SNI) are used.