Ever since social science institutions from 58 different countries included a 15-minute supplement to their national surveys. The membership to the ISSP is institutional and by country. One or more than one institute in a country can co-operate on ISSP research (cf. France and Spain). The common module surveyed by the member institutions also contains an extensive common core of background variables. The modules focus on one specific topic each year and were planned to be repeated more or less every five to ten years. When it comes to the researchers choice of topics, the relevance of the area of social sciences in the year of the survey is taken into account.