Table 2.

Results of the risk of bias assessment using the Newcastle-Ottawa Scale (NOS)

Author	Study design	Items for cohort studies								Items for case controls								Total NOS stars
		Selection				Comparability	Outcome			Selection				Comparability	Exposure
		Representativeness of the exposed cohort	Selection of the non-exposed cohort	Ascertainment of exposure	Demonstration that outcome of interest was not present at start of study	Comparability of cohorts on the basis of the design or analysis controlled for confounders	Assessment of outcome	Was follow-up long enough for outcomes to occur	Adequacy of follow up of cohorts	Is the case definition adequate?	Representativeness of the cases	Selection of Controls	Definition of Controls	Comparability of cases and controls on the basis of the design or analysis	Ascertainment of exposure	Same method of ascertainment for cases and controls	Non-Response rate
Brander et al. [18]	Cohort	*	*	*	*	**	*	*										********
Browne et al. [11]	Cohort	*	*	*	*	**		*	*									********
Mathews et al. [21]	Cohort	*	*	*	*	**	*	*	*									*********
Leivonen et al. [12]	Casecontrol									*	*	*	*	**	*	*		********
Motlagh et al. [22]	Casecontrol									*		*			*	*		****
Pringsheim et al. [23]	Casecontrol									*	*	*	*	**	*	*		********
Cubo et al. [24]	Casecontrol									*	*	*	*	*	*	*		*******