Table 4.

Quality assessment of the non-randomised studies (Newcastle–Ottawa scale)

Study	Selection				Comparability		Outcome			Score
	Representativeness	Selection of non-exposed cohort	Ascertainment of exposure	Outcomes of interest	Controls	Additional factors	Assessment of outcome	Follow-up	Adequacy of follow-up	Out of 13
Ervan et al. [24]	*	x	*	*	*	x	* record linkage	*	x	6
Shorter et al.[31]	*	X	*	*	*	X	*	*	X	6
Honjo et al. [17]	*	x	*	*	*	x	*	x	x	5
Staack et al. [18]	*	x	*	*	*	x	*	*	*	7
Katayama et al. [19], Japan 2011	*	x	*	*	*	x	* record linkage	*	x	6
Ueda et al. [27]	*	x	*	*	*	x	* record linkage	*	x	6
Lee et al. [25]	*	x	*	*	X	x	*	X	x	4

*Indicates that a feature is present; x, that a feature is absent. But for comparability by design this checklist awards a maximum of two stars (**), one (*) or none if the feature is completely absent (x)