Erratum to: Sexual behavior of migrant workers in Shanghai, China - Wei Dai; Jian Gao; Jian Gong; Xiuping Xia; Hua Yang; Yao Shen; Jie Gu; Tianhao Wang; Yao Liu; Jing Zhou; Zhiping Shen; Shanzhu Zhu; et al. (BMC, 2016)