National accounts or national account systems (NAS) are the implementation of complete and consistent accounting techniques for measuring the economic activity of a nation. These include detailed underlying measures that rely on double-entry accounting. By design, such accounting makes the totals on both sides of an account equal even though they each measure different characteristics, for example production and the income from it. As a method, the subject is termed national accounting or, more generally, social accounting. Stated otherwise, national accounts as systems may be distinguished from the economic data associated with those systems. While sharing many common principles with business accounting, national accounts are based on economic concepts. One conceptual construct for representing flows of all economic transactions that take place in an economy is a social accounting matrix with accounts in each respective row-column entry. National accounting has developed in tandem with macroeconomics from the 1930s with its relation of aggregate demand to total output through interaction of such broad expenditure categories as consumption and investment. Economic data from national accounts are also used for empirical analysis of economic growth and development. Scope National accounts broadly present output, expenditure, and income activities of the economic actors (households, corporations, government) in an economy, including their relations with other countries' economies, and their wealth (net worth). They present both flows (measured but it is over a period) and stocks (measured at the end of a period), ensuring that the flows are reconciled with the stocks. As to flows, the national income and product accounts (in U.S. terminology) provide estimates for the money value of income and output per year or quarter, including GDP. As to stocks, the 'capital accounts' are a balance-sheet approach that has assets on one side (including values of land, the capital stock, and financial assets) and liabilities and net worth on the other, measured as of the end of the accounting period. National accounts also include measures of the changes in assets, liabilities, and net worth per accounting period. These may refer to flow of funds accounts or, again, capital accounts. There are a number of aggregate measures in the national accounts, notably including gross domestic product or GDP, perhaps the most widely cited measure of aggregate economic activity. Ways of breaking down GDP include as types of income (wages, profits, etc.) or expenditure (consumption, investment/saving, etc.). Measures of these are examples of macro-economic data. Such