Table 1. Quality assessment according to Newcastle-Ottawa quality assessment for cohort studies.

Study/Quality assessment questions	Foulongne et al, 2013	Shabat et al, 2011	Malter et al, 1998	Iguchi et al, 2000	Kim et al, 2013	Kao et al, 2018	Lehto and Honkanen, 1995	Chang et al, 2005	Mannion et al, 2010	Lurie et al, 2015	Lee et al, 2014	Atlas and Delitto, 2006
1. Representativeness of the exposed cohort	*	*	*	*	*	*	*	*	*	*		*
2. Selection of the non-exposed cohort	*	*	*	*	*	*	*	*	*	*	*	*
3. Ascertainment of exposure	*	*	*	*	*	*	*		*		*	*
4. Demonstration that outcome of interest was not present at start of study
5. Comparability of cohorts on the basis of the design or analysis controlled for confounders	**	**	**	**	**	**	**	**		**	*	**
6. Assessment of outcome	*	*	*	*	*		*	*	*	*	*	*
7. Was follow-up long enough for outcomes to occur	*	*	*	*	*	*	*	*	*	*	*	*
8. Adequacy of follow-up of cohorts					*	*			*		*
	Low	Low	Low	Low	Low	Low	Low	High	High	High	High	Low

Note: This scale evaluates each study based on eight items, categorized into three groups: selection, comparability, and outcome. Each item numbered in the Selection and Outcome categories was given a maximum of one star and a maximum of two stars within the comparability category.