The International Standard Classification of Occupations, established by the International Labour Organisation (ILO), exists since the 1950s. The most recent version dates from 2008. In the meantime, the ISCO has been updated several times without changing its basic structure. The different versions can be converted into each other by means of a conversion key. The purpose of the ISCO is 1.) to provide a basis for international comparisons of occupations, 2.) to serve as a model for the development of national classifications and 3.) to provide an occupational classification in countries without their own (cf. http://www.ilo.org/public/english/bureau/stat/isco/index.htm).
The survey institutes or GESIS code the ISCO either directly from the full texts of the occupational information collected in the questionnaire or from the KldB by means of a conversion key.
Furthermore, it forms the basis for the creation of other occupational classifications, such as the Standard International Occupational Prestige Scale (SIOPS).