2. Methods
2.1. Design and sample
A cross-sectional survey design was used in the present study. A convenience sample of 600 full-time qualified nurses employed by five tertiary first-class hospitals in Tianjin was surveyed. The five teaching hospitals are in the highest Chinese health-care system ranking by the Ministry of Health. Of 600 questionnaires, 574 questionnaires were completed and returned. Our actual final sample consisted of 524 usable questionnaires with a response rate of 87%. Data were collected in March 2012.