Funding for this research was provided by:
Norges Forskningsråd (175361)
Received: 2 March 2018
Accepted: 3 October 2019
First Online: 22 October 2019
Ethics approval and consent to participate
: This study was conducted according to the guidelines of the Regional Committee for Medical Research (approval references REK 2009/788a and 2013/1585) and was approved by the Norwegian Data Inspectorate. All participants provided informed consent, and the identity of the respondents was not divulged to the researchers by the Central Bureau of Statistics Norway.
: Not applicable.
: The authors declare that they have no competing interests.