Table 2.

Quality assessment of cohort studies utilising the Newcastle Ottawa quality assessment scale

STUDY	SELECTION				Comparability		Outcome			SCORE (out of 13)
	Representativeness	Selection of non-exposed cohort	Ascertainment of exposure	Outcomes of interest	Controls	Additional factors	Assessment of outcome	Follow-up	Adequacy of follow-up
Gurol-Urganci[1] 2013	*	x	*	*	*	x	* record linkage	x	x	5
Prager[30] 2008	*	x	*	*	*	*	* record linkage	x	x	6
Bodner- Adler [31]	*	x	*	*	*	*	*	x	x	5
Buekens[32] 1985	*	x	*	*	*	*	*	x	x	6
Lam [33] 2006	*	*	*	*	*	x	* record linkage	x	x	6
Samarasekera,[24] 2009	*	*	*	*	*	*	*	*	x	8
Steiner[34] 2012	*	*	*	*	*	*	*	x	x	7
Andrews[35], 2006	*	x	*	*	x	x	*	x	x	4
Jango [11] 2013	*	*	*	*	*	*	*	x	x	7
Baghestan[36] 2010	*	*	*	*	*	*	*	x	x	7
Ampt,[37] 2013	*	x	*	*	*	*	*	x	x	6
Revicky, [8] 2010	*	x	*	*	*	*	*	x	x	6
Shihadeh,[38] 2001	*	*	*	*	*	*	*	x	x	7
Angioli, [39] 2000	*	*	*	*	*	*	*	x	x	7
Mora-Hervas [40] 2015	*	*	*	*	*	x	*	*	x	7
Twidale, [23] 2013	*	x	*	*	*	*	*	x	x	6

*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)