Received: 8 March 2018
Accepted: 9 September 2018
First Online: 20 September 2018
Ethics approval and consent to participate
: This study was approved by the Medical Ethics Committee of the Tokyo Medical University (No. 2762). Data were collected as administrative data by the Tokyo Metropolitan Board of Education and the Tokyo Metropolitan Government. The Tokyo Metropolitan Board of Education and the Tokyo Metropolitan Government approved the secondary use of these data for research purposes.
: Not applicable.
: The authors declare that they have no competing interests.
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