Table 2. The Newcastle-Ottawa Scale tool for risk of bias assessment of the selected studies.

For each criterion assessed using the Newcastle-Ottawa Scale, a notation of “*” is used to indicate that the study successfully meets the specified criteria.

Authors	Selection				Comparability of cohorts	Outcome			Total score
Prospective cohort studies	Representativeness of the exposed cohort	Selection of the non-exposed cohort	Ascertainment of exposure	The outcome of interest not present at the study start		Assessment	Length of follow-up	Adequacy of follow-up of cohorts
Styles et al. 2000 [18]	*	*	*	*	*	*	*	*	8
Styles et al. 2012 [19]	*	*	*	*	*	*	*	*	8
Ballas et al. 2006 [13]	*	*	*	*	*	*	*	*	8
Bargoma et al. 2005 [14]	*	*	*	*	*	*	*	*	8
Naprawa et al. 2005 [17]	*	*	*	*	*	*	*	*	8
Ball et al. 2012 [12]	*	*	*	*	*	*			6
Case-control studies	Is the case definition adequate?	Representativeness of the cases	Selection of controls	Definition of controls	Comparability of cohorts	Ascertainment of exposure	The same method of ascertainment for cases and controls	Non-response rate	Total score
Styles et al. 1996 [2]	*	*		*	*	*	*	*	7