a set of principles and methods used to measure the income and production of a country. There are basically two ways of measuring national economic activity: as the money value of the total production of goods and services during a given period (usually a year) or as the total of incomes derived from economic activity after allowance has been made for capital consumption.

The most commonly used indicator of national output is the gross national product (GNP), which is a measure of the total market value of currently produced finished goods and the value of services rendered. Because national output includes goods and services that are highly diverse in nature and some that are not in fact placed on the market, the determination of market value is difficult and somewhat imprecise. Nonetheless the use of a common basis of valuation makes it possible to obtain a total that fairly represents the level of output of a country. The rule that only currently produced goods and services should be counted ensures that only production occurring in the course of a given year is included and that any transaction in which money changes hands but no good or service does so in return (so-called transfer payments, e.g., unemployment or social-security payments, gifts) is excluded. The rule that only finished or final goods must be counted is necessary to avoid double or triple counting of raw materials, intermediate products, and final products. For example, the value of automobiles already includes the value of the steel, glass, rubber, and other components that have been used to make them.

National income may be derived from the GNP by making allowances for certain non-income costs included in the GNP, mainly the costs of indirect taxes, subsidies, and the consumption of fixed capital (depreciation). National income thus calculated represents the aggregate income of the owners of the factors of production; it is the sum of wages, salaries, profits, interest, dividends, rent, and so on.

The data accumulated for calculating the GNP and national income may be manipulated in a number of ways to show various relationships in the economy. Common uses of the data include: breakdowns of the GNP or the closely related GDP ( gross domestic product) according to types of product or according to functional stages in its generation; breakdowns of national income by type of income; and analyses of the sources of financing (depreciation; savings by individuals, corporations, or institutions; and national deficits).

In practice, statisticians face a number of difficulties and complications in computing the national product and income. Although there is a wealth of information available from regular production returns made by companies, from value-added tax figures, from income and corporation tax returns, and from other reports relating to incomes or expenditures, they are all incomplete, subject to errors, and based on different definitions and valuation methods. Statisticians have developed various techniques for estimating and adjusting so as to improve the quality of the figures. Much indirect evidence is used to close gaps in data. Margins of error that accompany published calculations are themselves subject to error. Thus simple comparisons of, for example, one nation's reported national product and income with another's may be misleading. National accounting remains an inexact science, but it constitutes an invaluable tool for economic planners and government budget makers.