Using the unique national identification numbers assigned to Swedish citizens as key,21 we linked several Swedish nationwide population-based registers. The National Patient Register (NPR) includes diagnostic information on individuals admitted to a Swedish hospital since 1969. From 2001, the NPR also contains data on outpatient consultations in specialized care.22 Diagnoses are based on the International Classification of Diseases (ICD), eighth (ICD-8; 1969–1986), ninth (ICD-9; 1987–1996) and tenth (ICD-10; 1997–2013) revisions. The Cause of Death Register contains a record of all deaths in Sweden since 1952, with compulsory reporting nationwide. Each record contains the date of death and codes for causes of death, also in accordance to ICD codes. The Cause of Death Register covers more than 99% of all deaths in Swedish residents, including those occurring abroad, resulting in minimal loss of information.23 Demographic and socioeconomic data were derived from the Swedish Register of Total Population with supplementy data from the Education Register.
