Funding for this research was provided by:
Ministry of Education, Culture, Sports, Science and Technology (15K16213)
Received: 27 October 2017
Accepted: 13 February 2018
First Online: 21 February 2018
Ethics approval and consent to participate
: The NDNS rolling programme is conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving human subjects are approved by the Oxfordshire Research Ethics Committee. Written informed consent is obtained from all participants.
: Not applicable.
: Both authors declare that they have no competing interests.
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