In the SNA, taxes are compulsory unrequited payments, in cash or in kind, made by institutional units to the general government exercising its sovereign powers or to a supranational authority and generally constitute the major part of government revenue in most countries. Social security contributions, which although being compulsory payments to general government, are not treated as taxes in the SNA because the receipt of social security benefits depends, in most countries, upon appropriate contributions having been made, even though the size of the benefits is not necessarily related to the amount of the contributions. However, many policy makers and users prefer to define taxes to include social security contributions. Indeed this is the basis of tax measures used in the OECD Revenue Statistics publication. This partly reflects the fact that the contributions to general government are compulsory but also because not all countries operate social security schemes, choosing instead to finance social benefits paid by government through other taxes or revenue (see also ).