Received: 22 November 2017
Accepted: 14 February 2019
First Online: 26 February 2019
Ethics approval and consent to participate
: Consents from the Ministry of Research and Education, the local school authorities, and the school boards were obtained in 1992 (T1) when the first data collection was conducted. Every participant gave a written consent based on both an oral and written description of the project formulated according to the standards prescribed by the Norwegian Data Inspectorate. According to these standards, a written informed consent was also obtained from the parents or legal guardians of participants below the age of 15 at the first data collection. Moreover, new informed consent was obtained in 1994 (T2) and 2005 (T4). The study was approved by the Norwegian Regional Committees for Medical and Health Research Ethics (reference # S-05030).
: Not applicable.
: The authors declare that they have no competing interests.
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