Table 3. Quality assessment conducted according to the NOS for all the studies included in the meta-analysis.

First author and year	Quality indicators
	Selection	Comparability	Exposure
Salimi et al. (2015) [16]	***	**	**
Joshi et al. (2014) [17]	***	*	**
Sinaga et al. (2014) [18]	***	**	**
Rathored et al. (2012) [19]	***	**	**
Singh et al. (2011) [20]	***	*	**
Kang et al. (2011) [21]	**	*	***
Sharma et al. (2011) [22]	***	*	**
Banoei et al. (2010) [23]	**	*	**
Marashian et al. (2010) [24]	**	*	***
Alagarasu et al. (2009) [25]	***	*	**
Selvaraj et al. (2009) [26]	**	*	**
Vidyarani et al. (2009) [27]	**	*	**
Merza et al. (2009) [28]	**	*	**
Selvaraj et al. (2008) [29]	***	*	**
Olesen et al. (2007) [30]	***	*	**
Bornman et al. (2004) [31]	***	*	**
Selvaraj et al. (2004) [32]	***	*	**
Selvaraj et al. (2003) [33]	***	*	**
Fibla et al. (2002) [34]	***	*	***

The number of asterisks indicates the number of criteria each publication is fulfilling for quality of ‘Selection’, ‘Comparability’ and ‘Exposure’. A study with a collective score of five asterisks or more is rated as good quality study.