Table 1. Assessment of quality and the risk of bias (NOS scale).

*Presence of criteria. ^aA score of less than 6 indicted potential risk bias and a score of more than 6 indicated a low risk bias.

NOS, Newcastle-Ottawa Scale

Description	Selection				Comparability		Outcome			Scorea
Study	Representativeness of the exposed cohort	Selection of controls	Exposure	Outcome interest not present at the start	Comparability of cohorts based on the design	Comparability of cohorts based on the analysis	Assessment of outcome	Was follow-up long enough for outcomes to occur	Adequacy of follow-up of cohorts
Doll et al. [10]	*	-	*	-	*	*	*	-	-	5
Chang et al. [11]	*	*	*	-	*	-	-	*	*	6
Colella et al. [12]	*	-	*	-	*	*	*	*	-	6
Koenigs et al. [13]	*	*	-	*	*	-		*	-	5
Lukits et al. [14]	*	*	*	*	-	-	*	*	*	7
Marocchio et al. [15]	*	-	*	*	*	*	-	*	*	7
Mohamed et al. [16]	*	*	*	*	*	-	*	*	*	8