Table 2.

Quality assessment according to the Newcastle–Ottawa scale

Eligible studies	Selection				Comparability		Ascertainment of exposure/outcome			Total score
	Item 1	Item 2	Item 3	Item 4	Item 5	Item 6	Item 7	Item 8	Item 9
Registry-based
Quach et al. (2014), USA	*	*		*		*	*	*	*	7
Li et al. (2010), Sweden	*			*	*	*	*	*	*	7
Herdt-Losavio et al. (2009), USA	*	*		*		*	*	*	*	7
Halliday-Bell et al. (2009), Finland	*	*		*		*	*	*	*	7
Axmon and Rylander (2009), Sweden	*	*		*		*	*	*	*	7
Hougaard et al. (2006), Denmark	*	*		*			*	*	*	6
Zhu et al. (2006), Denmark	*	*	*	*	*	*	*	*	*	9
Rylander and Källén (2005), Sweden	*		*			*	*	*	*	6
McDonald et al. (1988), Canada	*	*				*	*	*		5
McDonald et al. (1987), Canada	*	*				*	*	*		5
Questionnaire-based
Herdt-Losavio et al. (2011), USA	*	*	*			*	*	*	*	7
Ronda et al. (2010), Spain	*	*	*	*	*	*	*	*	*	9
Ronda et al. (2009), Spain		*	*	*		*	*	*	*	7
Peretz et al. (2009), USA		*	*				*	*	*	5
Gallicchio et al. (2009), USA		*	*	*	*	*		*	*	7
Baste et al. (2008), Norway		*	*	*				*	*	5
Axmon et al. (2006), Sweden	*	*		*	*	*		*	*	7
Kersemaekers et al. (1997), Netherlands	*	*	*	*	*	*		*	*	8
John et al. (1994), USA		*	*	*		*	*	*	*	7

For case–control or cross-sectional studies Item 1: adequate case definition, Item 2: representativeness of the cases, Item 3: selection of controls, Item 4: definition of controls, Item 5: control for the most important factor, Item 6: control for any additional factor, Item 7: ascertainment of exposure, Item 8: same methods of ascertainment for cases and controls, Item 9: non-response rate. For cohort studies: Item 1: representativeness of the exposed cohort, Item 2: selection of the non-exposed cohort, Item 3: ascertainment of exposure, Item 4: outcome was not present at start of the study, Item 5: control for the most important factor, Item 6: control for any additional factor, Item 7: assessment of outcome, Item 8: follow-up long enough for outcome to occur, Item 9: adequacy of follow-up of cohorts

* Positive score for the indicated item