- Correspondence to Dr Tingting Yao, Institute for Health & Aging, School of Nursing, University of California, San Francisco, 3333 California Street, Suite 340, San Francisco, CA 94118, USA;
- Received 9 September 2016
- Revised 29 March 2017
- Accepted 8 May 2017
- Published online 26 July 2017.
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