Received: 1 March 2020
Accepted: 29 June 2020
First Online: 6 July 2020
Ethics approval and consent to participate
: Ethics approval was received from the Regional Ethic Committee in Oslo, South-Eastern Norway, before the study started in 1993. As there was no Regional Ethic Committee register of projects at the time, there is no reference number. The study was re-approved by the director of the Regional Ethic Committee in South-Eastern Norway in 2015, Knut W. Ruyter.
: Not applicable.
: The authors declare that they have no competing interests.