Cohort Profile: the Swiss National Cohort--a longitudinal study of 6.8 million people.

For many years research on socio-economic inequalities in health in Switzerland was based on crosssectional data. Cross-sectional studies are problematic for several reasons. For example, results may be affected by numerator/denominator bias. Furthermore, occupational information from death certificates was used to describe the socio-economic position of individuals. However, this meant that those who do not work, older men and a substantial proportion of women, had to be excluded. Decennial censuses, conducted at the beginning of December every 10 years, have been done in Switzerland since 1850 (exceptions were 1890 and 1940, which were replaced by a census in 1888 and 1941, respectively). Deaths and causes of death have been registered since 1876, with data stored electronically since 1969. Death registration is anonymous. However, the date of death and birth are available, as well as gender, marital status, place of residence, date of birth of spouse and other variables. The 1990 census for the first time included the exact date of birth, which opened the possibility of linking census and mortality data. Based on the promising results of a pilot study that was done for one Canton at University of Zurich, the project was extended to cover the whole of Switzerland, linking the 1990 census with mortality records up to the end of 1997. The results indicated that linkage was less successful for foreign nationals and young adults, and led to the inclusion of additional data sources, including data on immigrants and emigrants and, importantly, the 2000 census. In 2005, an application by all five University Institutes of Social and Preventive Medicine (ISPMs) to obtain long-term funding for a Swiss National Cohort study (SNC) was successful within the framework of a Swiss National Science Foundation initiative to support longitudinal studies. Approval was obtained from the Ethics Committees of the Cantons of Zurich and Bern and a data centre was established at ISPM Bern.