Table 1.

The quality of the studies according to the Newcastle-Ottawa Quality Assessment Scale

References	Newcastle-Ottawa Quality Assessment Scale (NOS) score			Total score
	Selection	Comparability	Exposure/outcome
Abate et al. (2015) [41]	3	2	3	8
Abrahams et al. (2013) [23]	4	1	2	7
Brown et al. (2016) [36]	4	1	2	7
El Khoury et al. (2016) [38]	4	2	2	8
El Khoury et al. (2015) [35]	4	2	2	8
El Khoury et al. (2013) [24]	4	1	1	6
Gaida et al. (2009) [43]	4	2	2	8
Gaida et al. (2010) [44]	4	2	3	9
Gibbon et al. (2016) [37]	4	2	2	8
Hay et al. (2013) [25]	4	1	2	7
Longo et al. (2009) [42]	4	2	3	9
Mokone et al. (2005) [26]	4	2	2	8
Mokone et al. (2006) [27]	4	2	2	8
Nell et al. (2012) [28]	4	0	2	6
Owens et al. (2013) [40]	4	2	3	9
Posthumus et al. (2010) [29]	4	2	2	8
Raleigh (2009) [31]	4	2	2	8
Rickaby et al. (2015) [39]	4	1	2	7
Saunders et al. (2013) [30]	4	2	2	8
September et al. (2008) [34]	4	2	2	8
September et al. (2009) [33]	4	1	2	7
September et al. (2011) [32]	4	1	2	7