In practice, income definitions adopted by individual countries may be more limited in scope, as some elements of household income may not be collected or modelled (this is typically the case, for instance, with unpaid domestic services, with services provided household consumer durables, and by social transfers received in kind). The component elements of income can be aggregated in order to produce selected measures for particular analytical and policy purposes. Details of existing classification methodologies used for international comparison within the UNECE region are available from OECD (2011) and Eurostat (2015b).