Table 6.

Quality assessment using the Newcastle-Ottawa Scale.

References	Year	Country	Criteria
				Selection				Comparability			Outcome			Total score maximum
			Study design	1	2	3	4	5	6	7	8
Klingberg et al.	2007	Sweden	Cross-sectional			*		*	*					3
Lewyllie et al.	2017	Belgium	Cross-sectional			*			*					2
Heliövaara et al.	2011	Finland	Cross-sectional			*			*					2
Nordgarden et al.	2012	Norway	Cross-sectional			*			*					2
Klingberg et al.	2002	Sweden	Cross-sectional			*			*					2
Oberoi et al.	2011	USA	Cross-sectional			*			*					2
Ahonen et al.	1990	Finland	Cross-sectional	*		*			*					3
Bruserud et al.	2016	Norway	Cross-sectional			*			*					2
McGovern et al.	2008	Ireland	Cross-sectional			*		*	*					3
Ferre et al.	2016	North/South America	Prospective cohort			*		*	*					3

The use of

^*means that the publication qualified in the respective item of the Newcastle-Ottawa Scale. (1) Criteria. (2) Representativeness of the exposed cohort. (3) Selection of the non-exposed cohort. (4) Ascertainment of exposure. (5) Comparability on the basis of confounding control in the design or analysis. (6) Additional confounding control. (7) Assessment of outcome. (8) Duration of follow-up period.