Table 2.

Assessment of methodological quality

	selection			comparability		exposure		scores
	1	2	3	4	5	6	7
Wang 2013		*	*	*	*		*	*****
Petr 2013	*	*	*	**	**	*	*	*********
Miao 2013		*	*	*	*	*	*	******
Hofstetter 2013	*	*	*	**	*	*	*	********
QI 2013	*	*	*	*	*	*	*	*******