Table 2.

Newcastle-Ottawa Scale.

Study	Newcastle-Ottawa Scale			Overall Risk of Bias
	Selection (Total 4)	Comparability (Total 2)	Outcome/Exposure (Total 3)	(Total Score)
Takahashi et al., 1993, USA. [28]	***	-	***	Moderate (6)
Snow et al., 1993, USA. [29]	***	*	****	Low (8)
Sawle et al., 1991, USA. [30]	**	**	****	Low (8)
Tanji et al., 1998, Japan. [31]	*	*	***	Moderate (5)
Hanawaka et al., 1996, Japan. [32]	-	*	**	High (3)
Pal et al., 2002, USA. [33]	*	*	****	Moderate (6)

Newcastle-Ottawa Scale. Score: 0–3: High Risk for Bias, 4–6: Moderate Risk for Bias, 7–9: Low Risk for Bias. ** Good Quality: 3 or 4 stars in selection domain AND 1 or 2 stars in comparability domain AND 3 or 4 stars in outcome/exposure domain, Fair Quality: 2 stars in selection domain AND 1 or 2 stars in comparability domain AND 2 or 3 stars in outcome/exposure domain, Poor Quality: 0 or 1 stars in selection domain OR 0 stars in comparability domain OR 0 or 1 in outcome/exposure domain.