Table 3. Newcastle–Ottawa Scale for Observational Studies (cohorts).

Study	Selection	Comparability	Outcome	Total score
Denoyer et al[6] 2015	****	**	**	8
Li et al[14] 2013	****	*	**	7
Wang et al[16] 2015	****	**	***	9
Xia et al[17] 2016	****	**	**	8
Xu et al[15] 2014	****	*	**	7

A higher overall score corresponds to a lower risk of bias; a score of five or less (out of nine) indicates a high risk of bias. Each * equals 1 point.