Table 2. Quality assessment of the studies based on Newcastle-Ottawa scale.

Study reference	Selection Criteria 1	Comparability Criteria 2	Exposure/Outcome Criteria 3
Willershausen et al., 2011	***	*	**
Adegboye et al., 2013	*	*	**
Dodington et al., 2015	****	**	**
Luo et al., 2018	****	**	**

Maximum stars means criteria of good quality.

¹ Maximum of 4 stars.

² Maximum of 2 Stars.

³ Maximum of 4 Stars.

^*One star.

^**Two stars.

^***Three stars.

^****Four stars.