Table 4.

Risk of bias assessment of included observational studies according to the Newcastle-Ottawa Quality Assessment Scale

Author (year)	Selection				Comparability	Outcome
	Representativeness of cohort	Selection of nonexposed cohort	Ascertainment of exposure	Outcome of interest	Comparability of cohorts	Assessment of outcome	Adequate duration of follow-up	Adequate follow-up of cohort
Been and Bouma et al. [5] (1999)	*	*	*	*	**	*	*	*
Briem et al. [6] (2004)	*	*	*	*	**	*	-	-
Danisa et al. [7] (1995)	*	*	*	*	-	*	*	*
Mayer et al. [8] (2017)	*	*	*	*	**	*	*	*
Schmid et al. [9] (2012)	*	*	*	*	**	*	*	-

As judged by the Newcastle-Ottawa Quality Assessment Scale, maximum of one star awarded for each category within selection and outcome. Maximum of two stars awarded for comparability."