aggregate measures and their change over time are generally of strongest interest to economic policymakers, although the detailed national accounts contain a source of information for economic analysis, for example in the input-output tables which show how industries interact with each other in the production process. National accounts can be presented in nominal or real amounts, with real amounts adjusted to remove the effects of price changes over time.<ref>• Amartya Sen, 1979. "The Welfare Basis of Real Income Comparisons: A Survey," Journal of Economic Literature, 17(1), p p. 1–45. • D. Usher, 1987. "real income," The New Palgrave: A Dictionary of Economics, v. 4, p. 104.</ref> A corresponding price index can also be derived from national output. Rates of change of the price level and output may also be of interest. An inflation rate (growth rate of the price level) may be calculated for national output or its expenditure components. Economic growth rates (most commonly the growth rate of GDP) are generally measured in real (constant-price) terms. One use of economic-growth data from the national accounts is in growth accounting across longer periods of time for a country or across to estimate different sources of growth, whether from growth of factor inputs or technological change. The accounts are derived from a wide variety of statistical source data including surveys, administrative and census data, and regulatory data, which are integrated and harmonized in the conceptual framework. They are usually compiled by national statistical offices and/or central banks in each country, though this is not always the case, and may be released on both an annual and (less detailed) quarterly frequency. Practical issues include inaccuracies from differences between economic and accounting methodologies, lack of controlled experiments on quality of data from diverse sources, and measurement of intangibles and services of the banking and financial sectors. Two developments relevant to the national accounts since the 1980s include the following. Generational accounting is a method for measuring redistribution of lifetime tax burdens across generations from social insurance, including social security and social health insurance. It has been proposed as a better guide to the sustainability of a fiscal policy than budget deficits, which reflect only taxes minus spending in the current year. Environmental or green national accounting is the method of valuing environmental assets, which are usually not counted in measuring national wealth, in part due to the difficulty of valuing them. The method