Danish population-based registers for public health and health-related welfare research: Introduction to the supplement

Denmark and other Nordic countries have exceptional opportunities to perform register-based research, because of the unique personal identification number available to all persons with permanent residence [1]. This number makes it possible to link information at the individual level from several registers for investigation of various research questions. The unique personal identification number was introduced in Denmark in 1968, which enables follow-up of individuals for decades. This supplement of the Scandinavian Journal of Public Health presents public health and health-related registers and health-related welfare research based on Danish nationwide registers. The topics range from presentations of important registers, to introduction to the Danish legal foundations for register-based research, to short reviews of selected applications of registers for public health research. Linkage of the Danish registers is based on three base registers including identification numbers for persons, businesses, and real estates (Figure 1). All three base registers have linking keys to related registers and also to the other base registers. The linking keys are: the personal identification number (CPR-number) [1], the business identification number (SE/CVR-number) [2], and the building and housing identification number (BBR-number) [3]. The Civil Registration System contains the CPRnumber and includes references to parents and spouses, making it possible to establish the family unit. The Business Register contains the SE/CVRnumber of all businesses. The Building and Housing Register (BBR) contains the BBR-number, which identifies all unique housing unities. By the base registers it is possible to link persons, businesses and housing units. The aim of the supplement is to present a wide range of Danish registers and register-based research. We invited Danish key researchers performing register-based research to contribute to this supplement. Thus, this publication also highlights that registerbased research in Denmark is widely distributed at various research institutions and that registers are fundamental data sources in health and healthrelated welfare research. Twenty two institutes and departments at four Danish universities and 21 Danish administrative and research institutions contributed to the papers in this supplement. The supplement has three sections. The first section gives an overview of Danish registers, briefly introduces how to get access to data at Statistics Denmark and the legal foundation for register-based research. The section also presents an online database, which gives further information and an overview of the content of various Danish registers. This database also includes a search engine to help researchers to get more details on register contents. The second section includes presentations of important Danish registers on health and social factors. All papers have the same format, including four sections – in the Introduction section the background and rationale for establishing the register, the current purpose and historical pioneers are presented. In the Content section information on central variables in the register is provided and changes in variables and content, in reporting to the