"Denmark has developed a statistical system where a large portion of official statistics is based on administrative registers. Population and Housing Censuses have been replaced by register based statistics since 1981. The key of the system is the unique Person Number used in all public administrations. A large number of registers is used as sources and is linked together in order to compile the best estimates of the statistical concepts. This allows for high quality statistics that are timely, consistent and well-suited for longitudinal studies. Problems of quality may arise from changes in the legislation or in the registers. A strong data protection policy is necessary in order to preserve the system."