Fig 1. The Flow Chart of Sampling.

Three inclusion criteria were established in the selection of study participants: 1. The study participants should be aged 18 or older. 2. The study participants must sign the written consent. 3. The study participants must have visited a VC at least once in the past year. The interviewers were postgraduates and undergraduate students from the School of Health Management of Guangzhou Medical University. They were trained on how to conduct the survey in order to improve the completeness and consistency of the investigation. The interviewers were introduced to the participants by local acquaintances, and they could be village heads, women's directors, or the respected senior citizens. The data were collected through face-to-face interviews, and the questionnaires were administrated by the investigators at respondents’ home. It took about 20 minutes to finish the survey, and a gift was given to each participant as a token of appreciation for the participation.